A chance is a possibility — but it's not a sure thing. There could be a chance of rain today or a chance you'll be the lead in the school play.
Chance is sometimes about opportunity, as in getting a chance to try out for the baseball team. Sometimes it's just luck, as in taking a chance on a new car through a raffle at school. You could meet someone by chance, and you could say, "I chanced upon him at a coffee shop." Sometimes the word implies danger, as in "He took a chance when he signed up for skydiving." In any case, a chance is never a sure thing.