Depending on the intonation and circumstances, this can mean anything from, “where is everyone!?” to “look what I found,” to “stay away from me!”
My cat James is the King of Yowls. He loves to walk around the house yelling. Sometimes he sounds like a baby crying. Sometimes he sounds like a tornado siren. I’ve never heard anything like it. In older cats, nighttime yowling can be a sign of dementia, but since James has been yelling since he was a kitten (and at all hours of the day), I think he does it when he’s just woken up from a nap and needs to find a lap ASAP, like when you fall asleep on the couch and wake up to find everyone gone.
Joan and Fitz will often yowl when they bring us their favorite toys. This communicates that they want praise and playtime.
Angry yells and yowls can communicate distress, fear or pain. Again, pay attention to your cat’s intonation and body language. If your cat is yowling in a way that sounds angry, or like they’re trying to keep you away from them, this could be a sign that something is wrong.
While cat sitting, I’ve had cats yell at me when they feel like I’ve infringed on their territory. Not every cat is willing to accept love from a stranger, and that’s ok! In these instances, I do my best to take care of chores (food, water, litter, etc.) without bothering them. Disrespecting a cat’s boundaries is a sure way to stress them out, and I never want to do that.