The rating scale is a general analysis of the episode or film as a whole. There's no clear-cut application of a rating; it's just a useful estimate of my feelings about an episode or movie.
— Recommended with flaws
— Average, so-so
— Lower end of mediocre
or lower — Abysmal
Why aren't the ratings consistent?
The star ratings are not meant to be absolute, especially not between different series (and sometimes not even between different seasons of different series). It's a relative scale with a lot of uncertainty involved and factors that are not set in stone. I like to try to assign them with consistency, but that's probably impossible given the wide range of tones and intentions behind the different series/seasons/episodes/movies — not to mention my own changing feelings and attitudes spanning some two decades. It's an art, not a science.