What is meant by playing on adrenaline?

Does this mean when players aren't in game shape but are excited to be back so they're full of adrenaline and that allows them to keep up for the first couple games back?

I've had a couple games where I was really excited to play and I had way more energy in those games but I can't replicate that feeling every game. Has anyone else experienced this where some games you're pumped and have more energy than others?