Line bet has two separate meanings, one at the roulette table and one at the sportsbook.
In roulette, a line bet, also called a six-line bet, covers six numbers at once. You place your chip at the end of the line that divides two neighbouring rows of three numbers. A chip on the line between 4 and 7, for example, covers 4, 5, 6, 7, 8 and 9. It pays 5 to 1.
In sports betting, a line bet is a wager on a contest with two possible outcomes where the bookmaker adds a handicap to even out a big gap between the two sides. Your pick has to cover the line. If your basketball team is listed at −10 points, it must win by 11 or more for the bet to win. Both meanings share one idea: a line that defines the bet, drawn on the felt in one case and in the odds in the other.