Macbeth decides exactly what he will do at this point, ie. stand and fight. A resolution is a decision, and "pull in" gives an image of (for example) pulling in a net full of fish. So now he gathers his resources and acts. The doubt fdmaxey mentions is about the prediction of Birnham wood having a confusing or hidden meaning - Macbeth now realises the unimaginable (Birnham wood moving to Dunsinane) has happened exactly as predicted... the prediction was neither a lie nor a riddle (=equivocation). People don't speak like this nowadays. You could say it as a quote (meaning "Ok, enough messing about, time for a decision"), but I think even hardcore Shakespeare fans would miss the reference.

Hi... Please be careful don't try to speak like Shakespeare... It will be awkward.

Hi you can say, I am losing my courage to do something or you can say I'm too chicken to do that.

I believe that it means that Macbeth is losing his resolve (resolution, courage). In this scene, the trees are beginning to move out of Birnham wood because the enemy has cut them down and using them for cover. Macbeth believed the prediction that he would never be defeated as long as the trees stayed in the wood. I would not use this sentence - it is "Elizabethan" English and not too many of us still understand it easily these days.