What does "use work and movement" mean?

"Work" and "movement" are meta-tags designed for orchestral works - so that music like Beethoven's symphonies can be organized.

I simply checked the box to find out what happens.

You get a few additional rows on the "Get Info" display when checking the box:

Very likely, this is for the case where you have several albums to cover something like a long classical music piece.

While this looks useful for new tracks, I've been caught out trying to apply it to existing tracks as it deletes the existing title information. When you enter the "work" title for a group of tracks, the "movement" details are blank and you need to enter them from scratch, even if this information is already there in the track titles. As many of my classical tracks are vocal works whose titles often include foreign text with accented characters etc, this creates a lot of extra work. I think I'll pass on this new feature -- maybe the next update to iTunes will automatically transfer the existing title into the movement field...

Organize your classical music in iTunes - Apple Support

explains what you want to know in this official Article