Generally:

5 stars - Loved it. Will definitely re-read at some point, and will probably recommend it to anyone and everyone. (not rated many books this highly)

4 stars - Enjoyed it, will likely re-read at some point. Will recommend to people if it seems their thing. (My most used rating, as these days I pre-filter books before I read 'em because I know what I like!)

3 stars - Was readable, but was lacking something, there were large sections which were un-enjoyable, or it was just not what I was hoping for/not the quality I expected. Not getting re-read unless I have to for some reason. Also for books that were "not my thing".

2 stars - Readable, but just not good. Poorly written/formatted/edited, or just really not for me.

1 stars - Bad. Either almost unreadable, or the dreaded DNF. For books that I will not only not recommend, but will tell people to actively avoid (such as deliberately misleading or offensive books).