In sports betting, a "half time bet" is a bet that is placed on the second half of a sporting event.
For instance, let's say that the Golden State Warriors are playing the Oklahoma City Thunder in the Western Conference Finals. After a particularly strong first half, the Warriors are up by 20 points. Now, there is a very good chance that the Thunder will catch up a bit in the second half, as it is human nature to let your foot off the gas when you are up by a significant margin. In addition, the Warriors will likely use the lead as an opportunity to rest some of their top players.
Given these facts, a half time betting line for the game might look like this:
Oklahoma City Thunder Second Half, -7.5
Golden State Warriors Second Half, +7.5
With this line, the bookmakers are saying that the Thunder are expected to at least somewhat cut the lead in the second half.