A toss-up is a situation that doesn't have one outcome or choice that's clearly better or more likely than another. If it seems like either basketball team could realistically win, you could call that game a toss-up.
Your favorite ice cream flavor might be a toss-up between vanilla and strawberry. And if you two candidates for governor are running neck-and-neck, that's another kind of toss-up. The origin of this informal noun is unclear, but it may stem from the practice of tossing a coin to make a decision: "Let's do a toss-up to see who has to walk the dog. Heads I walk him, tails, you do."