Galvatron was a bad guy, of course. Now Nimoy is returning to the robot world, but as a good guy - Sentinel Prime, Optimus’ predecessor. I don’t know if that means he’s Optimus’ actual dad, like if he stuck his robot dick in a semi’s tailpipe or something, or if he’s just the guy who headed the Autobots before Optimus.
I’m certainly not going to bother trying to parse it, as I’m sure Transformers: Dark Of The Moon isn’t coherent enough for it to matter. What does matter is that Sentinel Prime, who is found by Apollo astronauts on the Moon in 1968, shows up as a firetruck at the end of the movie. That’ll be a cool toy.