An elision is an omission. In Macbeth, Shakespeare leaves all the murderous violence off stage and relies on reporting from other characters to let us know what has happened. The violence is narrated, not dramatized.

This begins as the play opens. In act one, scene two, we hear a report of Macbeth having cut the rebel Macdonwald in two from the neck to the nave, then placing his head on a pike. We don't see this happen. Later, we will learn about Duncan's murder—as well as the murder of his guards—from Macbeth's report to his wife. He will be horrified and discuss seeing enough blood to turn the earth's seas red, but the actual murder occurs offstage.

The same is true of Banquo. Macbeth will order his assassination, and later will see his ghost, but the actual murder takes place offstage. It is the same with Macduff's family and Lady Macbeth's suicide.

Shakespeare does this because he knows that actually dramatizing the murders—especially the murder of a king—would involve a level of gore that would possibly be disturbing to an audience. It is one thing to have a group of dead bodies at the end of a play, such as in Hamlet and Othello, and another to have repeated bloody murders dramatized throughout a play.

Why does Shakespeare use elision for key events in Macbeth?

Shakespeare deals with some events that would have been particularly gruesome and unsettling to his audience, so he elides them, which means he omits showing them to the audience. For example, murdering a king, especially a king that even Macbeth admits is good and just, would have been especially horrific to a Renaissance audience, who believed that monarchs were anointed by God to rule. It is best, for that reason, to keep Duncan's murder off stage. Killing Macduff's children would also, as it would today, be another deeply disturbing event to witness. By keeping these killings in the background, Shakespeare prevents his play from becoming overly lurid or gory.

Furthermore, Shakespeare wanted to keep the focus on the Macbeths and the terrible consequences the path of murder has on their lives. To the extent that this play is a cautionary tale, it warns the audience against becoming too ambitious. By keeping the spotlight on the way the Macbeths unravel in the face of what they have done, earning for themselves mental anguish and death, Shakespeare emphasizes the importance of ambition being tempered by a moral compass.

Elision in Macbeth allows for a few dynamics.  First, violent acts appear more grotesque when left to the viewer's/reader's imagination.  As the above response notes, many props are required to create a violent murder on stage, and traditional Shakespearean plays use minimal props.  So keeping the violence off-stage allows the viewer/reader to imagine the horror.  Next, elision allows space for a rapidly advancing plot to develop.  Macbeth is one of Shakespeare's shorter tragedies in terms of length; however, there are many events that happen in the timeline of the play.  Elision is used in part for efficiency.  Finally, elision also keeps the focus of the play on the characters and character development.  Viewers/readers may be distracted by violent acts, causing them to get wound up in the gruesome events of the play.  But the murder of Duncan, for example, in itself is not the important part--what is important is what leads Macbeth to kill someone whom he respects so deeply.  The themes and messages of the play are advanced through character development, so elision helps keep the focus where it belongs.

Your question regarding off stage action in Shakespeare's Macbeth is an interesting one.  It might be useful  first of all to consider which actions are completed off stage:  the slaying of Macdonwald, the execution of Cawdor, the murder of Duncan,  the murder of Lady Macduff, Lady Macbeth's suicide, and Macbeth's death.  In other words, most of the violence in this very bloody play takes place offstage, and this action is reported by other characters.  Logistically speaking, Shakespeare most likely had to omit some of these scenes because they would have been very difficult to stage.  His ability to use special effects was quite limited.  Further, it is quite likely that Shakespeare was influenced by the Greek tragedies of Sophocles and Euripides who also consigned violent acts to offstage, probably to maintain the three unities of time, place, and action.

Nevertheless, some of the reported action is more horrible in the imagining than in seeing it performed on stage.  Macbeth unseaming Macdonwald from his "nave to his chops" is one such example.  The execution of Cawdor is interpreted for us.  In other words, from the report of this death, we understand that Cawdor died nobly.

Nothing in his life became it like the leaving it.

And these words become quite haunting when we see Macbeth meet his own death quite courageously.  Both men, though, are traitors.

Yet, some gruesome or violent scenes are portrayed on stage:  the return of Banquo's ghost, the severed head of Macbeth.  Macduff's son is stabbed in full view of the audience--this act which takes place in broad daylight serves to highlight the depths to which Macbeth has sunk.  No summary dialogue or interpretative words are needed here, for the poignancy of this brutal act is best captured by the child's words:

He has killed me, mother.

So obviously it was not just logistics dictating the mode of the action.  In many cases, it seems to be the effect that Shakespeare desired to achieve.  We don't see Lady Macbeth's suicide perhaps because Shakespeare wanted to focus more on Macbeth's numb response.  We don't see Lady Macduff's murder but we hear the screams offstage and imagine the worst.  We don't see the actual beheading of Macbeth, but we do see Macduff holding Macbeth's head high in victory amid cheers of joy.