Miss Maudie is actually criticizing Mr. Radley (Boo Radley's father) for being a "foot-washing Baptist." In other words, he is a bit extreme in his religious beliefs and she even says that the Radley family cares more about the next world than this one.
At this point in the book Scout, Jem, and Dill have all been playing the Boo Radley game and Scout has decided she wants to stop for a while, which the boys don't agree with. This leads her to be left out of their play group for a bit and in the meantime she decides to hang out with Miss Maudie. It is then she learns more about the Radley family than she previously knew.
The point of Miss Maudie's criticism is to show Scout that Boo Radley comes from a difficult home. Essentially, if Boo didn't have some real issues of his own to begin with, then his father's fanatical religious beliefs probably created some for him. This chapter gives the reader, and maybe Scout a little bit, the chance to see that there are other factors at work in keeping Boo inside the house than are obvious.