What is an EM?
An EM is the unit used to measure size in typography by dividing the desired number of pixels to the parent size in pixels.
The formula for calculating this is as follows: if the parent size is 20px, then that will equal to 1 em. So, is for instance we have a font size of 30 px, then the em value will be 30px divided by 20px (1.5 em). This way of declaring sizes is manly used because the text can be scaled rather than having a static pixel value.