rem units (short form of  root-em) dictate the font size of an item relative to the size of the root item. By default, most browsers use a fontsize of 16 pixels. So, if the root element is 16px, an element with the value 1rem will also equal 16px.

rem units reference the size of the root element

Rem vs. Em Units

rem units are relative to the root element, em is relative to the parent element.