Being an introvert doesn't mean you don't like/need socializing, it means you get tired from it and need to recharge afterwards by being alone. Extroverts gain energy by being around others whereas introverts lose it
The usage of "energy" here is a metaphor for how the dopamine system is invoked in the brain. Introverts have less dopamine released in a social interaction than extroverts do, and that gives the impression of "tiredness" or "having less energy".
It's a feeling of depletion, but perhaps it's due to dopamine or another neurotransmitter