The coverage column on diffs has tooltips showing the meaning of the colors. However the coverage columns on file views don't seem to have these.
Revisions and Commits
Thanks for the report! I think D15719 should fix this; it should be available in master later today and stable in about 24 hours.
You may also find T9524 interesting, although it needs some additional infrastructure work before it can be completed. It will add an aggregate coverage summary view like this to directories: