When Jem shows up before a gathering of the neighbors without his pants, Dill makes up the excuse that he lost his garment playing strip poker. When Atticus, suspicious, asks if they were playing cards, Jem answers that they were only playing with matches. Scout thinks, "matches (are) dangerous, but cards (are) fatal". She means that to be caught gambling at all would invite punishment, but to be caught actually using cards would bring far more severe consequences. To be playing with matches, even when an element of gambling is involved, indicates a certain naivete and lack of sophistication , but to be gambling with actual cards is the real thing.
The children are in this predicament because Jem and Dill had decided to trespass at the Radley place to look in the window in hopes of catching a glimpse of the infamous Boo. Unfortunately for them, Mr. Nathan Radley saw them, and, not recognizing them as children, let off a shotgun blast to scare them off. When the young interlopers tried to flee, Jem caught his pants on the fence, and wriggled out of them to escape. When he, Scout, and Dill were confronted by the neighbors, Jem was embarrassingly pants-less, and had to quickly come up with an excuse for his situation. It is then that Dill offered that Jem lost his pants playing strip poker (Chapter 6).
The strong Puitanical attitude of some of the townspeople is echoed in Scout's words. Maycomb is located in the "Bible Belt" where playing cards and gambling are frowned upon as acts that harm people and destroy families. So, Scout simply repeats what she has heard in her cultural conditioning.
Harper Lee sets the cause of Scout's remark, the conditioned reaction of Miss Rachel who hysterically decries the "horrors" of card-playing, against the hypocrisy of the "good Christians" who are not in the least offended by the treatment of some of residents of their own town.
The kids are trying to evade any questions that will get them in trouble. They have just barely escaped being caught in the Radley yard, and Atticus is questioning them about what they were doing, and why Jem didn't have pants on. So, Dill did what he does best and lies about it, stating, "We were playin' strip poker up yonder by the fishpool." That would explain Jem's pants being gone, but strip poker is a rather scandalous thing for youngin's to be playing. Playing cards is considered by many to be a sin, including many people in the crowd that was gathered, listening to Dill's story. So, Atticus tries to clarify by asking, "Were you all playing cards?" Jem knew how down on cards people in that crowd were, so he stepped in by responding, "No sir, just with matches." It is then that Scout admires his brother's response, because according to Atticus and many of the folks listening, "matches were dangerous, but cards were fatal". This simply means that adults thought playing with matches was a dangerous thing, but playing with cards was fatal to your eternal salvation. She understood the adults and their attitudes well, as did Jem, and that is what got them out of the scrape this time.
This is a funny line in Chapter 6, and it gives us an idea of the values with which Scout and Jem were being raised in Maycomb. While stumbling around to find a reason for Jem's pants being missing, Dill says they had been playing strip poker, of all things. Gambling! To save Dill from Miss Rachel's wrath at the very idea of their gambling, Atticus intervenes and asks if they had been playing cards. Jem says that they had been playing only with matches. Scout then says, "Matches were dangerous, but cards were fatal." She means that the children would have gotten in trouble for playing a game with matches, but they would have gotten in BIG trouble if they had been gambling with cards, surely a serious matter in Maycomb at this time. Card playing was considered a vice by many people.