Gregory and Sampson are about to fight with the Montague servants. Sampson is detailing how he would bring the Montague castle to its knees.
One of the ways he says he will do this is by either beheading the maids or taking their maidenheads, which is their virginity. While he seems to be full of bravado, his threat is either to kill or rape the Montague maids.
He uses the play on words of the maid's head or their maidenhead to be clever.
A maiden is defined as an unmarried woman. Particularly in Elizabethan times, this would mean that the woman is also a virgin. So, the term maidenhead refers to a young woman's virginity. If you are having trouble defining some of the text, consider using the etext provided by enotes, which gives a side by side reading in traditional Elizabethan and a more updated English.
sam. 'tis all one I will show myself a tyrant: when I have fought with the men, I will be cruel with the maids; I will caught off their heads.
gre. The heads of the maids?
sam. Ay, the heads of the maids, or their maidenheads; take it in what sense tou whilt.
gre. They must take it in sense that feel it.
In this scene the humor is derived from the wordplay of a maiden's head (literally the heads of Montague women) and the maidenheads of the Montague girls (ie their hymens). This might seem gruesome but whole scene is humorous banter.
gre. To move is to stir, and to be valiant is to stand; therefore, if thou art moved thou runnest away.
sam. A dog of that house shall move me to stand: and I will take the wall of any man or maid of Montague's.
gre.That show's thee a weak slave for the weakest goes to the wall.
The scene even begins with a hotheaded Sampson stating he and Gregory will not be insulted as low or base "we'll not carry coals". Gregory who seems calmer and good humored responds jokingly, taking Sampson's comment literally. Sampson then explains he means if they are insulted (an we be in choler=angry), they will fight (we'll draw...our swords). Gregory replies with the advice "ay while you live, draw (wordplay on drawing out a sword) your neck out o' the collar (hangman's noose), insinuating that if they fight, they could be hanged. Sampson says then, he'll strike quickly, being moved (being provoked). Gregory suggests that Sampson is a coward "thou art not quickly moved to strike" which is prooved in the next scene
sam. My naked weapon is out. Quarrel, I will back thee. (I've my sword out. You go fight, I'm right behind you)
The wall which they talk about is the highroad. Back in the day, the streets were really nasty so the nobility and women were give the part of the walk closer to the houses (the wall) whereas the servants or men took the side closer to the road. Therefore "I will take the wall of any man or maid of Montague's " means I am better than anyone in Montague's household. Gregory retorts that the weakest go to the wall because women, old people, ect. are offered the wall. To end an explanation of the scene:
sam. they shall feel while I (my penis) is able to stand; and 'tis known I am a pretty piece of flesh( well hung) Gregory responds it is good that you are not a fish, for if you were you would be the shriveled poor John fish (you are not well-hung). Hey get your sword out the Montagues are coming.
The whole scene is just a comedic way to open the play. It is really just a fancy locker-room conversation between the 2 servants . hope this helped clear it up.
In Elizabethan times a maidenhead would be a virgin. Women who were unmarried where expected to be virgins.This was all part of the punning that takes place at the opening of the scene. In all probability the comment was more bravado than a potential outcome.