Good evening,
The REM value is derived from the "Base Font Size" setting defined within your theme:
CP -> Design -> Standard Themes
If you edit your theme you will see the setting in the "Fonts" section. Consider a REM value a multiplier of that value. So if you have a "Base Font Size" of 16px, and you want to target a element to use a 12px font size you would set the REM value to 0.75rem. That is 16px * 0.75 = 12px.
I think I can sense what you are thinking: "But Jonmark, why wouldn't you just set the font size explicitly to 12px?"
If you were thinking that question great, if not, then I apologize because I am throwing a answer at you anyways:
The reason you would use a rem value over a explicit px value is because if your user has their browser set to increase/decrease the base font size then the calculated value for that user may be more or less than 12px. For example, some users may have poor vision and need to use a larger font size, so their browser could be set to use a base font size of 24px, which would result in a 0.75rem value of 18px.
This helps ensure that users see the site based on their needs, while providing you the ability to configure things to a base style that you think works best.
Does that help?
Jonmark