Hot steams is a supersition. Jem explains it to Dill in chapter 4:
"Haven't you ever walked along a lonesome road at night and passed by a hot place?" Jem asked Dill. A Hot Steam's somebody who can't get to heaven, just wallows around on lonesome roads an' if you walk through him, when you die you'll be one too, an, you'll go around at night suckin' people's breath."
The children are on summer vacation and they are trying to figure out a game to play. In front of the Radley house, Dill says "I smell death" and then he proceeds to tell how it is possible to know when someone is going to die. This conversation brings up the topic of hot steams. When Jem tells Dill what hot steams are, Scout contradicts him, telling him that they are not real, and that Calpurnia has told the children that hot steams "is nigger talk." Jem gets angry about this, so when the children decide to play with an old tire, Jem is too big to get into the tire, so Scout gets in, Jem pushes her too hard because he is angry, and she winds up rolling right in front of Boo Radley's house:
The tire bumped on gravel, skeetered across the road, crashed into a barrier and popped me like a cork onto pavement. Dizzy and nauseated, I lay on the cement and shook my head still, pounded my ears to silence, and heard Jem's voice: "Scout, get away from there, come on!"