Was changing the faucet in my shower and the old faucet was connected to the water lines with a brass washer looking thing with threads on the inside and on the outside. Don't know what this is but I need to get one with the same O.D. And a little bit smaller I.D. For the new faucet.

Do you mean something like this?

That brass washer looking thing with threads on the inside and on the outside.is called a bushing. (see Harlods image) Just take the old one into a Plumbing Shop and ask for a replacement. Don't forget to use Teflon Tape on the threads, Good luck, Tom