Odds against a selection indicate that the bookmaker believes there’s less than a 50% – or less than “even” – chance of the selection winning.
For example, odds of 4/1 indicate that a bookmaker will pay you four times the amount you stake on a bet if the outcome you’ve backed occurs. If the outcome doesn’t occur, the bookmaker keeps your stake. In other words, the bookmaker is betting against the outcome occurring.
Odds in which the amount you can win (excluding your stake) is lower than the amount you stake are known as “odds on".