Jesus Power is what Christians believe animates their religion and gives them special magic powers. It is this power that makes miracles happen, changes hearts and minds, and drives evangelism.
Typically, a Christian taps into Jesus Power through prayer and fasting. (Usually the method is prayer, since pretty much nobody fasts anymore.) In my Pentecostal days, we called the state of having Jesus Power being prayed up. We thought of it like a gas tank, which prayer and fasting filled up. As we went through our days, we depleted our tank much like driving a car depletes a real gas tank's contents. Praying for stuff, especially miracles, depleted it extra.
When a Christian is feeling on fire, that's an expression of feeling euphoric thanks to what they imagine is Jesus Power. (I jokingly call that state the Jesus Aura as well.)
Of course, nobody's ever actually credibly confirmed the existence of any supernatural being or state. Nor has anybody ever confirmed that Jesus Power is anything but suggestibility and a heaping dose of the placebo effect.
When Christians feel the full weight of Jesus Power, you can expect them to do some really wacky things. This is where you find stories of them walking up to motorcycle gangs and preaching at them, or deliberately traveling to super-hostile foreign countries to spread the Gospel, or trying to resurrect dead people in public settings.
After such a Christian deconverts, they usually end up eating a lot of crow about those episodes.
It's okay. We've probably almost all been there. The important thing is to keep learning and growing.