css unit which is not standard measure in CSS px, em, cm, code example

Example 1: what is em in css

'em' units for the font-size property will be relative to the'font-size of the parent element'.
'em' units on other properties other than font-size will be relative to the
font-size of the current element. 'rem' units sizes will always be relative 
to the font-size of the root html element.

Example 2: css em

em -> Relative to the font-size of the element 
(2em means 2 times the size of the current font)