In Macbeth Act II, Macbeth's aside shows that he is jealous of Malcolm and wants to "O'erleap" him in status. Macbeth wants to be next in line to be King, not fourth. Macbeth says:
The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be,
Which the eye fears, when it is done, to see.
This is the contraction of two words: "over" and "leap." To fit his syllable count, Shakespeares shortens and fuses words. So, "O'erleap" simply means "to leap over. "
Macbeth knows that he can't become King with Malcolm (and Donalbain) in the way. Therefore, he will kill Duncan and blame his sons, specifically the newly crowned Prince of Cumberland, Malcolm. In this way he can lead over both princes to take the crown.