UK bookmakers traditionally display fractional odds (5/1), but almost every site lets you switch to decimal odds (6.0), which many punters find easier to compare. Decimal odds show your total return per unit staked, including your stake.
Odds also express implied probability. Convert decimal odds by dividing 1 by the odds: 2.0 implies a 50% chance, 4.0 implies 25%. Comparing the bookmaker's implied probability with your own assessment of an event is the foundation of value betting.
Because different bookmakers price the same event differently, keeping accounts with two or three sites and taking the best available price is one of the simplest ways to improve long-term results.