What does the first murderer mean when he tells Macbeth we are men?

Macbeth is building up a case that Banquo has caused injury to the murderers, so the murderers, in attacking him, are fulfilling some kind of moral duty. So he outlines the injuries the murderers have suffered and the First Murderer says, perhaps a little ironically, "You made it known to us." As in, we didn't know that we were so hard done by until you told us.

Macbeth responds by asking, "Do you find your patience so predominant in your nature that you can let this go?" This is a roundabout way of calling them cowards, the kind of guys that people can insult with impunity.

The Murderer responds "We are men, my lord." As in, "What do you think? Any real man would respond to such offences."

This is all part of the "Real Man" dialogue that runs throughout the play. Remember Macbeth in Act 1 scene 7: "I dare do all that may become a man; who dares do more is none."