“Energy” is typically heuristic shorthand for “overall patterns of moods, attitudes, and behavior.”

If someone’s behaving differently towards you (general “you” here), regardless of it having to do anything with their relationship to you, you’re going to pick up on it largely subconsciously.

Because the human brain works on shortcuts and said subconscious processing, it typically translates to “idky just vibes I guess” even though it’s a lot of pattern recognition and social calculations.