I see this “em” frequently what does it mean?

Question

“In addition to percentages, you can use pixels or ems.” What are ems? (Could have been explained in a one of the lessons and I missed it)

Answer 1

https://developer.mozilla.org/en-US/docs/Learn/CSS/Building_blocks/Values_and_units#Relative_length_units

Answer 2

See the Relative Lengths section in the following page

https://www.w3schools.com/CSSref/css_units.asp

Answer 3

An em is a CSS unit that measures the size of a font, from the top of a font’s cap height to
the bottom of its lowest descender.

Answer 4

thanks to every one, direct answer or a link all is very valuable to me and helping me build my knowledge :)