Usually the monitor runs at a set refresh rate regardless of what the gpu can do. (No freesync or gsync)
With Freesync the gpu is able to tell the monitor to slow down its refresh rate to match the amount of frames per second the gpu is sending.
A screen tear happens when the monitors refresh rate is a non-whole fraction of the frames per second being rendered.
For example if you had a 60 hertz monitor and you locked your fps to 30 hertz you wouldn't see any frame tearing.
The action won't be particularly smooth but you shouldn't see screen tearing, you would just show every frame twice.
60/30 = 2 no tearing
Now if you had a 60 hertz monitor and you locked your fps to 45 hertz you would be seeing tearing like crazy due to every frame having the last 33% of it having to wait to be displayed till the next screen refresh.
60/45 = 1.33
Edit: had to correct math in last example.
https://www.techspot.com/article/1668-how-many-fps-do-you-need/
Has a nice visual of that above.
