What are em and rem font sizes?

Ever wonder what 1em or even a 1rem means when you are looking at your WordPress theme’s stylesheet and thinking what the hell is that? Normally when we look at font sizes, many people often think of pixels (px) as a unit of size. In the past, this was the standard…until we started evolving into accessibility. So what I will do here is give a simplified description of the differences between em, rem, %, and px, but I will try not to get too technical to descibe it.