Objectification is focusing on a person’s usefulness to you with total disregard for their desires. In the context of compliments, it’s not saying “You turn me on.” It’s saying “You turn me on, and whether you want to turn me on is utterly irrelevant.”
Saying “nice ass” to a person who’s deliberately wiggling their ass at you is a compliment; saying “nice ass” to a person who’s just walking by is objectification. “I want to sleep with her” is expressing desire; “I’d hit it” is objectification. “You’re sexy” is nice to say on a date because it’s a compliment; “you’re sexy” is hideously undermining to say at a business meeting because it’s objectification.