Mrs. Henry Lafayette Dubose is an extremely nasty old lady who lives in Maycomb. She is really abusive towards Jem and Scout.
Mrs. Dubose seems to yell at the two kids mostly because she really strongly disapproves of what their dad does with his law practice. She says that he represents "trash and niggers."
At one point, Jem trashes her flowers in retaliation for her abuse. Atticus makes him go and read to her every day as punishment.
After she dies, we find out that she was battling a morphine addiction and that she finally beat it and died in pain. Atticus says that what she did is truly brave.