Implied probability is the win rate that a set of odds represents — the break-even percentage you'd need to hit for the bet to be profitable. It's simply 1 ÷ the decimal odds.
Every price is really a probability. Decimal odds of 2.00 imply 50% (1 ÷ 2.00). Odds of 1.50 imply 66.7%; odds of 4.00 imply 25%. Convert every price to implied probability and you can compare bets, spot value, and measure the vig.
The implied probabilities a book shows include its margin, so both sides add up to more than 100%. To get the fair probability, you have to devig the market first.
Implied probability = 1 ÷ decimal odds. For American odds, convert to decimal first, then divide.
Because of the vig — the sportsbook's built-in margin. The excess over 100% is the overround.