There are a couple important events in Xen, but it’s also a very surreal and different game experience, so it’s very worth it.
Personally I think you should buy Half-Life: Source and play that through to the end. It’ll give you Xen, plus an appreciation for the original.
There are also mods that add fan-made Xen ports to Black Mesa if you wanna try those. There’s one that’s been going around on the BM workshop that seems to be really popular. But that’s only available for the retail Steam release of the game, so if you’re playing the free version then you’ll definitely want to play the original, because the free version won’t be getting Xen anyway.
Welcome to the community though! If you can ignore the need for Half-Life 3, and also ignore the people complaining about it, you’re in for an experience that’ll change how you feel about first-person shooters.
As an aside, I also recommend you play Portal if you want a little extra lore, it’s in the same universe and it’s also a really good game.