Every price carries an implied chance. Convert the price to a percentage and you know how often the bet has to land just to break even, which is the number to compare your own opinion against.
Implied probability is the chance an outcome has according to its price. The sum is simple: divide one by the decimal odds. A 2.75 shot implies 1 ÷ 2.75 = 36.4%; a 2.00 shot implies 50%; a 4.00 shot implies 25%. That percentage is also your break-even strike rate — the share of bets at that price you must win just to stand still.
The number matters because it is what you compare your own opinion against. If a price implies 36.4% but you honestly rate the selection a 45% chance, the price is in your favour and the bet has positive expected value. If your estimate is lower than the implied chance, the price is not worth taking, however tempting the payout looks.
Add the implied probabilities of every runner in a market and you get more than 100%. That surplus is the bookmaker's margin — the overround — and it is the edge you have to overcome to win in the long run.