Monitors basically work by displaying a series of still images on a screen. A Monitor's refresh rate is rate at which that monitor can change the image being shown. This is measured in cycles per second, or Hertz (Hz). A 60Hz monitor can change it's display 60 times per second. A 144Hz monitor can change it's display 144 times per second. You get the point.
The higher the frame rate, the smother motion looks on screen. However, your monitor will not display more frames than it is speced to, so if your PC is rendering a game at 100fps, a 60Hz monitor will still only show you 60fps. This is why everyone is asking you if you have actually used a high refresh rate monitor before assuming that you can't see any difference above 60Hz.
That being said, not everyone's eyes are the same, so it's possible you won't get much out of using a high refresh rate monitor. if your eyes can see the difference, then you should see much smoother animations when running games above 60fps on a monitor that supports that.