The circulation pump, as its name suggests, makes your water circulate through your spa, and guarantees the proper filtration and cleaning of your water. Often not that powerful, they ensure the filtration cycles of your spa or continuous filtration. They have to be capable of renewing all of the spa's water in a relatively short amount of time.
24-hour circulation pumps are quieter and more efficient than 2-speed pumps, and keep the water cleaner.
Note: If you want to replace your pump, checking the brand of your hot tub will not give you the appropriate information. Note the dimensions of the pump and/or wet end to be replaced and, most importantly, the information noted on the label on the motor (or more rarely the wet end).