When said by a human to another human, it may mean:
1. If I were a dog right now, i'd probably hump your leg.
2. I want to "pounce" you.
3.
I'm in heat...
let's fuck.
4. I have a concealed boner
with your name on it.
5. I am trying to lure you into bed
You see someone you are attracted to, but
you don't know what to say to
lure them into bed, so you just
purr at them