CSS Font-size EMs

The EM size unit is a relative size unit that is equal to the base body’s font size, which will be set by you or the browser.

So if the body’s size is 12px then 1 em = 12px. If you set the body’s font-size to 14px then 1em = 14px;

EMs are used because they are supposed to scale better on diferent monitor sizes.