An REM (Root Emphemeral Unit) is a unit of measurement relative to the font size of its the root element, which is the element. If the element has no font-size set, the browser default of 16px is used.
For example:
If the base font size is 16 px and a font in the document is set to 1.5 EM, the font is 16 x 1.5, or 24 pixels.
If a parent element’s font is changed to 20px (assuming it not the root element), then a font of 1.5 rem will still be 16 x 1.5, or 24 pixels.