Act 3, scene 6 of William Shakespeare's Macbeth is a very short scene of little more than fifty lines sandwiched between two scenes involving the Witches.
The first Witch scene, also very short, involves Hecate, the leader of the Witches, who scolds the Witches for interacting with Macbeth and making prophecies to him without her.
Many scholars believe that this scene with Hecate, which is often cut from modern productions of Macbeth, wasn't written by Shakespeare at all, but might have been added by another playwright at some time after 1605/06, when Shakespeare wrote Macbeth, and before the publication of Macbeth in the First Folio in 1623.
The scene directly after act 3, scene 6 is the famous "Double, double toil and trouble; / Fire burn, and cauldron bubble" scene involving the Witches, Macbeth, and several apparitions who make further prophecies to Macbeth.
Act 3, scene 6 involves only two characters, Lennox and a Lord. Lennox is a nobleman and warrior who has been suspicious of Macbeth since the moment he and Macbeth went together to look at the body of the murdered Duncan at Macbeth's castle.
In this scene, Lennox and the Lord are standing outside Duncan's former castle at Forres, and Lennox is basically thinking out loud to the Lord about what has occurred up to this point in the play.
It occurs to Lennox that anybody who had any serious dealings with Macbeth is dead, but Macbeth has somehow escaped any suspicion or consequences for those deaths.
Duncan's death was first blamed on his guards (who were conveniently killed by Macbeth), then on his sons, Malcolm and Donalbain. The irony is that the audience knows that Macbeth killed Duncan, and that Lennox's suspicions about Macbeth are well-founded.
Banquo's death is blamed on his son, Fleance, who fled the scene of Banquo's murder, much as Malcolm and Donalbain fled Scotland after their father's death. Again, the audience knows that Macbeth ordered the murder of Banquo and Fleance, but that Fleance barely escaped with his life.
Lennox and the Lord's reference to Macbeth as "tyrant" rather than "king" are a clear indication that both of them suspect that Macbeth had an "accursed" hand in all of these murders.
The Lord tells Lennox that Malcolm has fled to England, and that Macduff has followed him there. They've persuaded the English King to join them against Macbeth, and plans are being made to invade Scotland, remove the "tyrant," restore order to the "suffering country," and relieve Scotland's people from Macbeth's oppression.
In time, Lennox and his soldiers will join with Malcolm and Macduff against Macbeth
Nevertheless, Macbeth needn't worry. In the "Double, double toil and trouble" scene that follows, Macbeth learns from the Witches and the apparitions that "[N]one of woman born / Shall harm Macbeth" (4.1. 88-89), and that "Macbeth shall never vanquish’d be until / Great Birnam Wood to high Dunsinane Hill / Shall come against him" (4.1.103–105).
By the end of the play, the irony of those prophecies will also become clear.