There are mainly two types of ways to define sizes in CSS; absolute and relative. Absolute units are any measurement where the size will retain the same size no matter what; regardless of the other elements on screen, and screen size. The only absolute unit that should be in use is pixels, or px.

The other, relative, change based on a few factors. When using text, you’ll want to use rem or em, but most likely just rem. When using rem for font size, you’re setting the font size of the root element, which means as scale increases so does your text.

Viewports also receive a unit of relative measurement, vh and vw, for viewport height and width respectively.

Link for all CSS units. Link for overview on units. Link for more info on viewports.