Fixed odds is the standard form of betting at bookmakers. When you accept the odds displayed at the moment of placing a bet, those odds are locked in and do not change regardless of subsequent market movements. If you take 3.0 on a football team and the price later drops to 1.80, you still settle at 3.0.
This gives bettors certainty and planning ability. You know exactly what you will receive if your selection wins — there is no uncertainty about the settlement price. For accumulators, this is especially important: each leg is locked in at the moment the bet is placed.
The contrast to fixed odds is starting price (SP) betting, where your wager settles at whatever odds the market shows at the moment the event begins. If you think a horse will shorten significantly (become more popular), taking an early fixed price is advantageous. If you think it will drift (become less popular), taking SP is better.
Fixed odds form the foundation of most sports betting markets worldwide. The bookmaker sets the price, incorporates their margin, and accepts bets at that price. They manage their position by balancing the book or adjusting prices as money comes in. The fixed-odds model contrasts with betting exchanges, where prices are set by the market participants and settlement can happen on a floating price basis.
Example
You back a horse at fixed odds of 5/1 (6.0 decimal) the night before a race. By race morning, public money has pushed the horse in to 7/4 (2.75 decimal). Because you have fixed odds, your bet settles at the original 5/1. A £10 stake returns £60, rather than £27.50 at the morning price.