Em and Rem units are used to size your text elements relative to one another. By assigning em units to your elements, you can change all of their font sizes at once, simply by changing the font size of the elements' root tag. With rem units, you can change all elements' font size by changing the default font size of your root <html> tag.
These two units come in very handy if you ever need to scale things up or down as your site grows. When to use each is a little tricky, so let's learn a little more on how to use each with the above video.
