In many church denominations, playing cards are considered inappropriate, because they are often tied to the vice of gambling, and because the cards themselves often have inappropriate pictures on them. So, using playing cards was often look down on, especially if you were a young child. When Jem tells his lie about playing cards (a lie to cover the fact that they were almost caught sneaking into Boo Radley's yard, and almost got shot by Mr. Radley), he quickly asserts that they were playing poker "just with matches". The funny thing about this exchange (found near the end of chapter 6) is that Jem told all of the crowd that they were playing "strip poker". The crowed "stiffened", but they seemed more disapproving of the use of cards than the fact that little kids were playing strip poker. It's an amusing observation on some of the strange social rules that exist and people's perceptions of them.
I hope that helps; good luck!