One-to-one odds (1:1 odds, 50:50, or 1-1 odds) means that the two choices have an equal chance of happening. Flipping a coin, generally, is seen as having a 50% chance of landing on either heads or tails and is then 1:1 odds. 5000-1 odds would mean that one thing is much, much more likely to occur, with one option being 5000x more likely than the other.