in the soliloquy macbeth says.''if it were done when 'tis don,then 'twere well it were done quickly'',what is he talking about?

Maccbeth is talking about the killing of Duncan. 

He is contemplating the killing of Duncan, and is finding reasons not to do it. Here's my attempt at a translation:

If only the whole thing could be over and done with once he's murdered; in that case, I suppose it would be best to do it soon. If only murdering Duncan didn't have any consequences and his death could bring me the success I crave (kingship). If only this murder could be the beginning and end, and I could jump the river of worldly consequences that will surely come between the assassination and its rewards (power, prestige, fame). But in this life, there are consequences. In fact, if a person commits a bloody crime, like the one I'm considering, the act ends up serving as an example for other people to do the same to the original criminal. The things we do seem to come back around to us, like a poisoned drink that we will eventually sip from ourselves.

Macbeth then goes on to say that he shouldn't do it because he's related to Duncan, he's his subject, he's his host, and Duncan has been a really good king. He resolves not to do it, until his wife comes in and calls him unmanly for changing his mind and being timid.