A blow out is where you hair is straightened using the blow dryer with a concentrated nozel and a round brush. The combination of heat and holding the hair taut provides very straight results.
It's usually accompanied every haircut I've ever gotten at white salons, versus a roller set or marcel flat iron at a black salon. Dominicans are the best (that I've tried). They are cheap and the perfectly frizz free hair lasts for several days. Dominican salons are known for providing super straight, shiny, and bouncy hair via blow outs and they are more familiar with hair that has a natural curl pattern.
There are several how-to guides on Youtube for blow outs. I've found it a lot more difficult to straighten a whole head of hair myself using a blowdryer and brush than I imagined. I've honestly not perfected a blow out yet.