Yeah, you're honestly better off reading up on things. Even doing some
wiki reading should help some things make sense. There's still a lot that hasn't been explained due to the story itself being incomplete compared to even the first Halo, but there's still quite a bit there that just wasn't explained in the second game.
And yeah, I enjoyed the first game for what it was, but I definitely wouldn't go out of my way to play through it. While shooting still feels as good as ever, the RPG elements are
insanely grindy compared to D2, like to the point where you can hit the level cap in WoW before you max out a single subclass in D1 (that's not even an exaggeration!). D1 feels like an old school MMO in terms of that--progression, even after the revamp that happened in The Taken King, feels glacial when you get to a certain point. D2 does literally everything better.