Change Text Size tool bar?

In some website, there is a small tool bar type of thing that has 3 different size “A”s on it. By clicking on the “A” you change the font size of the site. What is this called?

And is this tool bar available as a WordPress plugin?

Basically, I need to implement it on a WordPress site, but I couldn’t find out what it is called in the first place … please help.