What is the meaning of "Man Hu"?
Rashi: It means a preparation of food.
Rashbam: It is the Egyptian term for "What is it?", the actual expression used by the people.
As in Daniel, 1:5 - "Va'yeman lahem ha'Melech" (and the king prepared for them [Rashi]).
Which explains why the Torah sees fit to add "because they did not know what it was", in the way that it tends to do when it uses foreign words. See for example Bereishis 31:47, and Megilas Esther, 3:7, 9:26.
What did Moshe mean when he responded "This is the bread that Hashem has given you to eat"?
Targum Yonasan: He was telling them that this was the bread that Hashem had designated for them from the time of the creation.