When you second-guess something, you either predict what will happen in the future, or look back and reevaluate something that happened in the past. You might second-guess your decision to wear shorts after it starts snowing.
You can second-guess how your favorite player will perform in the Super Bowl, but you won't know until you watch the game. And then you might second-guess the way your team played. The first use of second-guess has you guessing what someone will do. The second, which is more common in the U.S., comes from baseball slang, "for a fan who loudly questions decisions by players."