Like the title states.

 

From what I understand (I've never been a big drag racing fan, but I love cars so I read on them all the time), the "trap speed" is the average speed of the car in the last 60 feet of the 1320. It is determined by the time it takes the car to go the last 60 feet divided by the (known) distance. It is one of the reasons that cars can turn the same ET or the same "trap speed" and one car still wins; a car may be geared for acceleration so that it has a low 1000 foot time, but is slowing in its acceleration at the end, another may be all top end, and their trap speeds might be identical, with different ETs, 60 foot times, 1/8 times, etc.