Green 'progress bar' in Database view - what is it?

I have just noticed (but it has probably been there for years) a green sort of 'progress bar' which looks as though it is related to the size of the table in the Database view. This appears on all tables bit is bigger the bigger the table. It worries me a little as some of them appear close to 'full'. Does anyone know what this means?

See here.

It isn't a progress bar, it's a graph. So isn't "full", it's "greatest" ;-)

Many thanks - all makes sense now