A "Total" or "Over Under" NBA bet is a wager on the total amount of points in a given half from both teams combined.
With a total you will either be betting "over" which means you think the total points in a half exceed the line or "under" which means you think the combined score will be less than the number posted for the 1st half of play.
Hawks vs Suns 1st Half Total of 115. 1H Score ATL 65 PHO 60. In this example the first half total line was 115 points which means you are wagering if the combined score will be over or under that number. With 125 total points in this 1H the Over bettors would win this wager.