I have no spur to prick the sides of my intent, but only vaulting ambition

To remember this quote use the following mnemonic.

That man has pricked the side of my tent but I have no spurs to prick the side of my tent. 

But I am brilliant at vaulting.

It's my ambition to vault a really big tent one day. You could say it's my vaulting ambition.

"I have no spur to prick the sides of my intent, but only vaulting ambition." Act 1, scene 7, line 25.

This line is spoken by Macbeth himself. Macbeth is contemplating the murder of King Duncan but he is torn between his ambition to be king and his moral hesitation to commit such a heinous act.

"I have no spur to prick the sides of my intent". Here Shakespeare is using a metaphor comparing spurring a horse forward and spurring his plan to murder forward. Macbeth is saying he has no reason to move forward or go ahead with the plan to murder. 

"But only vaulting ambition". This is another metaphor where Shakespeare again uses a horse and rider who leap over an obstacle but this could lead to falling down. This is saying that Macbeth does have the ambition to leap into being a king but this could also lead to his own downfall.

This quotation reveals Macbeth's inner turmoil and moral conflict and foreshadows the tragic consequences of Macbeth's vaulting ambition. His only motivation to kill King Duncan is his overpowering ambition despite his moral reservations.