To predict is to say what you think is going to happen in the future. If you predict that you'll win the poker championship, you're either really confident in your poker skills or you're cheating.
The prefix pre means "before." Dict comes from the Latin dicere, which means "to say", yet you can use predict to refer to things that can't say anything at all. Darkening skies can predict a coming storm, for example, and a runny nose can predict a cold. You can predict something based on factual evidence, or on a crystal-ball reading, or just on plain intuition.