Odds represent the probability of an event occurring and determine the potential return on a successful bet. In the UK, you will encounter two primary formats.
Fractional odds (e.g. 4/1 or 5/2) are the traditional UK format. The number on the left represents potential winnings relative to the stake on the right. A £1 bet at 4/1 yields £4 in winnings plus your £1 stake returned.
Decimal odds (e.g. 5.00 or 3.50) are commonly used on mobile platforms. They represent the total return including your stake. A £1 stake at 5.00 returns £5 in total.