The hook means a half-point in sports betting.
For example, if you want to buy the hook, you would buy a half-point in the spread. If you're betting the Patriots -7.5 (seven and a hook) against the Bills, you would buy the half-point and turn that into -7.
In another instance, if you want to bet the Dolphins +3.5 against the Jets, you could buy the hook and move the line to +3 to guarantee you get your money back if the Dolphins lose by three points (a field goal).
This is a useful strategy, mainly in football when spreads are near common numbers like 3, 7, 10, 13 or 14. While buying the hook reduces the payout, it also boosts the chances of getting money back. If you took the above Patriots bet at -7.5 and they win by seven points, you're out of luck. Instead, if you bet the hook, you would push and get your money back.