This screen shows you the width of the browsers that are displaying your site to users. At the top you have a simple graph of the 3 main types (Desktop, Tablet, and Mobile) and then there is a table with a more granular breakdown by pixel widths.
Why it matters
It can be helpful to be aware of the split between screen sizes of your visitors. Roughly 50% of internet traffic is on mobile, 46% is on desktop.
How to use the browser sizes screen
If your visitors are not roughly even, you may want to load up your site on different devices and have a look. Designers often make it so that at certain widths the layout of the website will change, so you may want to experience it for yourself.
The more granular breakdown in the table can be particularly helpful with this, as there is often a difference between the experience on say, an iPhone 12 mini and an iPhone 12 Pro Max.