Energize means to raise the energy level of something. If your really boring teacher suddenly starts throwing M&Ms to every student with a correct answer, he will energize the classroom.
Ever seen the Energizer Bunny––the one who "keeps going and going and going?" It's because he's energized by long-lasting batteries that he has the energy to beat that drum over and over. Someone who's feeling low on energy might be reenergized by a cat nap or a candy bar.