They learn that Calpurnia's congregation in To Kill a Mockingbird are human beings, no different than most others. Although it is unusual to see white faces in the church, Jem and Scout are treated well. They initially have a run-in with Lula, who objects to their presence, but they are made to feel at home for the remainder of the service. The children learn that the congregation is poor and mostly illiterate; there are few hymnals since they can't afford them and, besides, most of the members cannot read. So, Calpurnia's son, Zeebo, leads them in "linin'": he reads the line, and the congregation repeats it. The children also find that the members are supportive of Tom Robinson. Although they are quite poor, the collection plate--actually, a coffee can--is passed around repeatedly until Reverend Sykes determines that enough money ($10) has been collected. In the end, Jem remarks that Reverend Sykes is "just like our preacher."
Scout and Jem learn quite a few things.
First, they learn that the black church is much poorer than their own. They see that they don't have hymn books and church bulletins and such. They probably should have figured the black church would be poorer, but they didn't.
More importantly, they learn that some parts of the black community resents white people and wants them to stay in their own place. Also importantly, they learn that the black community expects its members to act like they are black -- this is why Calpurnia speaks like a black person while she is among other black people.