In Act 3, Scene 1, Macbeth says, "There is none but he/ Whose being I do fear, and under him/ My genius is rebuked, as it is said Mark Antony’s was by Caesar." (III.1.57-60). Macbeth is speaking of Banquo, who he fears because the witches' prophecy predicted that Banquo, who would never be king, would be the father of kings. Macbeth says that his future promise will be negated, just as Mark Antony's was by Caesar.
This is a reference to Shakespeare's play Antony and Cleopatra, in which Antony asks the soothsayer in Act II, Scene 3, " Whose fortunes shall rise higher, Caesar's or mine?" (line 19). The soothsayer answers, "Caesar's./Therefore, O Antony, stay not by his side" (lines 20-21). The soothsayer, who predicts the future, tells Antony that the angel that guides him to his future will be weaker than the angel that guides Caesar, so Anthony must not stay with Caesar. Similarly, Macbeth fears that by staying with Banquo, his angel will be overwhelmed and his future contests will be lost to Banquo. Macbeth therefore resolves to kill Banquo. This reference reveals that Macbeth thinks of himself as if he were a great leader from the ancient world, such as Mark Antony, and that he is willing to challenge the witches' prediction--and fate--by slaying Banquo.
It would be helpful to us if you could quote the passage you have in mind. It's also helpful to provide act, scene, and line numbers, although, with Shakespeare, usually the quotation alone will help someone find the precise place in the text.
Presumably the passage you have in mind is this one:
There is none but he
Whose being I do fear: and, under him,
My Genius is rebuked; as, it is said,
Mark Antony's was by Caesar.
Here, Macbeth is referring to his fear of Banquo, whom he considers a possible rival. The allusion to Mark Antony and Caesar suggests that just as Antony was eventually defeated by Octavius Caesar, so Macbeth fears that he may lose in a contest of fortunes with Banquo.