What does Hester mean when she says,"Once i have met the Black Man? This scarlet letter is his mark."

This is interesting. The term "Black Man" is really a play on words. Pearl had heard women talking about a "Black Man" and she interprets him as the Devil. Hester, however, is referring to Dimmesdale. He is a minister and wears black. His "mark" was his sin meaning that Pearl was probably consummated in the forest they were in. He "sin" eventually leads to the A on Hester's chest. So Hester tells the truth about her version of the "Black Man" who made his mark; she just tells it in a way that Pearl can understand (without the sexual details).