So Hamlet's father was king until some guy named Claudius came along, killed him, married Hamlet's mother, and took over the throne. Hamlet is, by custom, supposed to avenge his father's murder by going ahead and killing Claudius himself. But, for whatever reason, he just doesn't do it.
The thing that critics have been puzzling over for ages is: why the heck can't Hamlet just get on with it and kill Claudius?
Let's look at the passage below. It's a soliloquy by Hamlet in which he's kicking himself for being such a procrastinator. Hamlet's just finished watching a play, and he compares himself to the actors ("players") in the scenes he's just finished watching.