High energy potential food sources is another way of saying big prey for trout. A trout is far more willing to expend energy to get a large piece of food like a damsel nymph or a cicada than it is to get a tiny caddis nymph or a small mayfly.
The size of the meal means it is worth the trout using a lot of energy to get the meal. Where a small fly may not be worth moving across or up in the water column to, a larger meal is worth moving to.
Where I fish, the East Coast of the North Island of New Zealand, there are five large food sources that trout take aggressively. These are Damsel Nymphs, Cicadas, Freshwater Crayfish, Bullies and Earthworms. If I chose to fish in the estuaries I would add Smelt to this list, but estuary fishing often means fishing blind or at night, neither styles of fishing I enjoy.
I have yet to perfect a bully pattern, so I have only written about the four flies I use a lot, the Leech, Earthworm, Koura & Madam X. These represent the high energy potential food sources in the areas I fish, and have consistently created an very aggressive response from the fish I fish to.
In other waters Damsel Fly dry flies, Sculpin, Minnows, Smelt, Bullies, Mice, very large Mayfly, Frogs, Tadpoles and other large food sources can attract aggressive takes from trout. I do not cover these patterns as I do not fish them, so cannot explain how to tie and present them.