Both mean “are you?”, but Welsh has two forms of addressing people: ti is for people you are familiar with, friends, pets and children. chi is for formal situations and/or people you don’t know. Those are the general guidelines, you’ll learn the difference over time.
…and there you have it, @JonWest. I too am new to SSIW so have only been around 5 minutes in comparison to the 5 minuter’s and all
I don’t remember vocative from Now You’re Talking (but perhaps not many of us might) and my lovely Cymraes was only just divulging the generalities to me.
Also a word of warning - a friend of mine who is first language welsh said chi/ti thing can be a tricky area. She used chi to refer to a colleague because they were in a work setting but she said by the look on the other persons face she interpreted it as chi because of her age.
Hopefully people will be a tad more forgiving on us poor learners!