Joe throws Pip's indenture papers in the fire and lets them burn.  The reason for this is that Pip is given the chance to become a gentleman - Mr. Jaggers, an attorney, has come to tell Pip and Joe that a mysterious benefactor has given Pip a great deal of money, as long as he follows certain rules, including never inquiring as to who exactly the benefactor is.  Pip is to accompany Mr. Jaggers to London where his education as a gentleman will begin.

Mr. Jaggers offers money to Joe in exchange for Pip's apprenticeship.  Because the apprenticeship is a legal contract, a master would be entitled to some payment if his apprentice was being taken away.  However, because Joe cares so much for Pip, he refuses to take money for him.  He is actually quite put out that someone would assume that he would take money for Pip, or otherwise stand in the way of Pip's chances to improve his lot in life.

This scene is important in furthering Joe's characterization in the story.  Despite Pip's embarrassment of him, wishing Joe could be more refined, Joe continues to be good, kind, gentle, and eternally loving and supportive of Pip, regardless of Pip's future airs as a gentleman.

Check the link below for a summary of this part of the book.  Good luck!