The vig (also called juice or hold) is the margin a sportsbook builds into its odds. It's why the implied probabilities of both sides add up to more than 100% — the extra is the book's cut.
Fair odds on a coin-flip market would be 2.00 on each side (50% + 50% = 100%). Books instead offer something like 1.91 / 1.91, where each side implies ~52.4%. The two add to 104.8% — that extra 4.8% overround is the vig.
Overround is how far the implied probabilities exceed 100%. Hold is the share of balanced action the book expects to keep. Lower vig means prices closer to fair — and better long-term value for you.
Add the implied probabilities (1 ÷ decimal odds) of every outcome. Anything over 100% is the overround; the hold is 1 − (1 ÷ that sum).
Lower-vig markets give you prices closer to the true odds, so more of your bets are +EV and you keep more of your winnings over time.