Matthew Pocket is Miss Havisham's cousin and the only member of her family who spoke his mind about her fiancé, Compeyson. He told her he was not a true gentleman and that he had serious doubts about the proposed marriage, so obviously she ordered him out of the house. But now she misses him, and claims that she wants him at the head of the table when she dies.
Pip goes to "read" with (a.k.a. study with) Mr. Pocket at his house in Hammersmith. Mr. Pocket is a great scholar who makes a living selling books (about things like raising children), lecturing, and teaching others. Pip respects Mr. Pocket hugely and asks Miss Havisham to leave him lots of dough in her will.