What advice is Faber offering Montag when he says "If you hide your ignorance, no one will hit you and you'll never learn?"
Faber makes this statement in Part Two of the novel when Montag has just read aloud the poem "Dover Beach." Realizing that this action was a mistake, Montag feels foolish and stupid and, in response, Faber uses this quote as a means of reassurance. In fact, in Faber's experience, it is far better to be open and honest about one's thoughts and feelings (whatever they might be) than to suppress them and always do as people expect. This is what he means by the phrase "hide your ignorance." In addition, Faber says that it is important to receive criticism from others because this will help Montag to refine those thoughts and feelings. This is what he means by "no one will hit you and you'll never learn."
So, although Montag is very worried that reading the poem was a bad idea, Faber encourages him to realize that there are some advantages. Even if he is "hit" (punished) by society, it is important to express his true feelings so that he can better understand his goals and ambitions.
Faber says this to Montag through the two-way earpiece. Montag has just read the poem, "Dover Beach", to Mildred and her two friends. One of the friends started to cry so Montag says that maybe he's wrong and books really do make people unhappy. Montag says maybe it would be a mistake to try to bring down the system as he and Faber have planned. Faber tells him that a person learns by making mistakes. He says that when someone makes a mistake, that person is figuratively hit by people who don't like the mistake. He goes on to say that the person who made the mistake learns to avoid making that mistake again in an effort not to get "hit" again. Furthermore, Faber says that a person's skills become sharpened through repeated attempts and repeated efforts to avoid being "hit". He says to hide one's ignorance just to avoid trying something and possibly making a mistke, keeps one from learning. In other words, Faber says we learn by making mistakes.