Reckon means guess, or imagine, and is often used by rural types in Hollywood movies who say things like "I reckon I'll be moseyin' on."
Reckon means guess or think, as in "I reckon he's put his nose where it don't belong one too many times.” If reckon sounds odd, that’s because it's mostly gone out of style. When used to talk about prediction, it's a little more common, as in "Who do you reckon is going to win the Super Bowl?" Still, it sounds a little old-fashioned. Many people use the word just for fun. Saying "I reckon" sounds more humorous than "Yes."