what is under her bed?
I am not positive of this answer, however, I think the answer to your question is a boy named Charles Baker Harris (Dill). Dill, who is supposed to live in another town with his "new father" who hasn't paid enough attention to him, runs away and comes to Maycomb and Scout finds him hiding under her bed.
I assume you are talking about Scout. She steps on something and thinks it's a snake under her bed. Instead, she and Jem discover Dill has run away from home. The Finch children convince Dill to tell Atticus what he has done.