Pip told this to Biddy because in this chapter, he notices that she is happier and more settled, and even a bit pretty. He realizes that she has been a good friend to him, a good friend and teacher, and he recognize clearly the difference between her and the cruel, cold and beautiful Estella. He realizes, when they are at the table one night, that "perhaps I had not been sufficiently grateful to Biddy," and he resolves to make her more of a friend, and to treat her better. She had done a lot for him, teaching him, caring for his sister and Joe, and he felt grateful for it. So, he asks her for a walk and a talk the next week.
Biddy is a natural listener, and Pip has known her for a long time. He feels perfectly comfortable with her, and that he can tell her anything. Along these lines, he opens up and confesses his miserable mind-set to her. In talking to the comforting Biddy, he confesses of Estella,
"She's more beautiful than anybody ever was, and I admire her dreadfully, and I want to be a gentleman on her account."
So, despite the fact that Estella makes him miserable, he is still in love with her, and wants to live a life that is worthy of her. Biddy is very kind in receiving his confession, giving him a comforting shoulder and wise advice. She remarks that he really shouldn't care what Estella says about him, and Pip admits that she is right, but can't help it. I hope that those thoughts help; good luck!