Refresh rate is the amount of times the screen refreshes per second. Something most gamer’s are familiar with (PC gamer’s at least) is FPS, or frames per second. A monitor’s refresh rate is it’s maximum supported FPS. So if a monitor has a native 144Hz refresh rate, it will refresh the screen 144 times per second.
While there is not much discernible difference between say 165Hz and 240Hz to the average human observer, the real gains are made when playing a game at something like 60Hz vs 120Hz. Game-play will feel markedly smoother, and for gamer’s wanting maximum advantage – getting more frames per second is always better than less. If you are playing a game at 240Hz vs your competitor at 120Hz, you will experience double the frames in a second allowing you to make split second decisions easier. This is why pro-gamers prefer higher refresh monitors, among other reasons.