I've never played a game in this series and know nothing about it. Is this an action-RPG or something else? Does it help to have knowledge of previous games, or can this stand on its own?
Action-RPG.
Typically the series is more focused on action (think: exploring a maze-like map with real-time combat) than on RPG, although has many standard RPG tropes built into the story. Lacrimosa is apparently a bit heavier in the RPG side of things - but take that with a grain of salt. I've only played the introduction before going back to finish Ys: Memories of Celceta.
The games stand extremely well on their own. There is an over-arching world you are in, and some characters (mostly just the hero) crossing between games, but pretty sure you don't need any of that knowledge at all to enjoy playing any single game in the series.
Won't be everyone's cup of tea... and personally I found the old 2D entries in the series pretty mediocre. But the more recent games are very accessible and should have wide appeal. Check out some of the PS4 reviews if you want to see firsthand.