The general theory of relativity is based on the concept of curved space–time. To describe how the energy and momentum of fields are distributed in space–time, as well as how they interact with the gravitational field, a special mathematical construct is used—the energy–momentum tensor. This is a kind of analog of energy and momentum in ordinary mechanics.
Just to fill out a bit of framework:
All matter and energy in the universe prefers to flow towards the laziest possible resting position between several different points of equilibrium, and stay there. Everything can be explained mathematically in the context of these symmetries, such as charge, space and time. There are several more. This is the #1 main concern of physics.
When something seems to be violating a symmetry, it is assumed that there is a deeper, more fundamental equilibrium at play. This is how they came up with The Strong Force (symmetry of something they call “color” because it behaves exactly like the RGB color scheme) and The Weak Force (symmetry of something they call “spin” even though nothing is really spinning, it just behaves like it is).
“Conservation of energy” is one of these symmetries, and it states among other things that a stick of dynamite will explode with the same release of energy yesterday, today and tomorrow. A crucial symmetry of energy is through time.