You can read it like a distribution graph rotated 90 degrees. I just pull the site's Best XI ratings for all 20 clubs when the summer transfer window closes. This season, we had a 27.35, 27.10, 25.55, 23.70, and so forth all the way down to an 11.25.
The histogram just charts out those values in a way that lets you visualize the distribution of ratings to a degree. You can see there's clear separation between the top 4 and the rest of the league, a mode in the 13s, etc.