It's just an integer from 1 to the maximum number of threads your computer can run. So if you have quadcore AMD or dual core intel with 4 hyperthreads, you can run '-t 4' to use max threads, or '-t 1' to use just one.
threads = cpu i want use ?
Usually a smaller number ends up being slightly better. Also if you want to use the computer for something else or keep heat buildup down (especially on a laptop) you should use a smaller number. You have to experiment with your setup to find the best.