Why can’t her mother explain what it means?

Hester's refusal to tell Pearl the true meaning of the letter is symbolic of Pearl's role in the novel. Pearl has often been compared to a living version of the scarlet letter. Thus, until she is told what the letter really means, she is unable to know herself. Her role as a living scarlet letter is to announce to the whole world who the guilty parties are, something she has unwittingly done throughout the novel.