I learned in a video that cats can read our facial expressions just fine, we're just crap at reading theirs because their facial muscles don't allow for the same movement as humans (and dogs to some extent). They'll become more anxious if we show a fear expression around them in a new environment, or become more relaxed and cuddly if we smile at them in a new place. They look to us for reassurance as much as dogs do.
Once I learned that they do a lot of their communicating with their tail, I started paying attention to my two cat's tail movements and now I can't unsee it. It's as obvious as a waving hand, and they'll talk to one another this way as well as with us.
For example, they lift their tail as a greeting. If I say their name as they enter a room, I might think they'd completely blanked me if I didn't see their tail lift 'hello' every single time. Once my older cat, Bartine, didn't bother to tail lift, and I said "Oi! Barty! Rude?!". She then gave me a quick, half-hearted lift, like she couldn't be bothered with more than half wave, lol
Their tails quiver with excitement if there's a very interesting treat up for grabs, or my favourite is a coquettish swirl which is 100% "I love you" because it's always followed up with an approach to snuggle or headbutt. They also understand me when I say I love you, but particularly now because I see the swirl tail and say "I love you, too!" followed by indulgent pets.
Additional unfun fact, in case the implication goes by anyone; some few folks have discovered exactly how much it sucks when your immune system discovers your eyes and have, indeed, gone blind because of it :(