A pun is a play on words through use of double meaning that is used for humorous effect. In a tragedy it adds lightness to a dark scene and is known as “comic relief.”
In the beginning of Act 3, Macbeth is talking to Banquo about the banquet that night.
We should have else desired your good advice,
Which still hath been both grave and prosperous
In this day's council; but we'll take tomorrow. (3:1)
Macbeth’s use of the word “grave” here [grave: serious] is a particularly good pun because he is planning to kill Banquo and put him in his grave.
The best pun in Act 3 appears after the murderers return to Macbeth.
My lord, his throat is cut; that I did for him.
Thou art the best o’ the cut-throats! Yet he's good
That did the like for Fleance. If thou didst it,
Thou art the nonparell.(20) (3:4)
Macbeth uses the fact that the murderer cut Banquo’s throat to make a pun about the murderer being cut-throat, which means ruthless.
In a dramatic and tragic play such as this, there needs to be some little jokes to cut the tension. In fact, the play is quite humorous throughout because of puns. Other instances of examples of comic relief puns are with the porter in Act II and the reaction to Banquo’s ghost later in Act III. The puns allow the audience to fully experience the drama through cutting the tension every once in awhile.