Vig (short for "vigorish") is how sportsbooks make money. On a standard -110/-110 two-sided market, the implied probabilities sum to ~104.76% — that 4.76% is the book's margin. To beat the book, your edge has to be large enough to overcome the vig.
Basics
Vig (juice)
The sportsbook's built-in margin — the reason implied probabilities add up to more than 100%.
Example
At -110 both sides, you need to win 52.4% of bets just to break even.