Is remastered mainly just less jitter?

I have heard a number of remasters that are definitely not improvements over previous masters. So frequently the term remaster simply means crap shoot.
On the other hand remasters can improve things immensely, the Beatles and Springsteen catalogs are good examples.
I have no way of knowing if jitter plays a role in either scenario.

also the 2002 or so re-masters of the Stones 1980s releases are supposed to be MUCH better

Most of the re-masters I’ve heard are just louder and more compressed.

Typical example was the Moody Blues On the Threshold of a Dream, the original Decca release is so much better than the MFSL re-master.
Sure the Decca was a "little" raw, but the MFSL re-master was blunted and fat, no excitement in comparison.

Cheers George

The Stones remasters in hybrid cb/sacd were excellent and much better than prior releases. Those remasters provided the greatest improvement over original redbook in the rock domain that I have ever heard. Remasters in general are a crapshoot.

I concur w/ The Rolling Stones on SACD (remastered), as well as, The Police on SACD, Van Halen (CD) and Rod Stweart (CD).  Otherwise, it is a crap-shoot.  Happy Listening!