The natural and man made systems on Earth that produce energy can only produce a set amount, a maximum. It receives only so much sunlight, oil wells produce only so many barrels, hydroelectric projects have a set amount of water flow and production.
The limits on the amount of this available energy affects everything else we can do. It is part of our carrying capacity, affecting how much food we can grow, how much transportation and electricity is available, etc. So the energy budget of Earth is one of the limits that humans have to live by.
From a purely scientific viewpoint, the term Energy Budget reflects all natural incoming and outgoing flows of energy. geothermal, tidal, sunlight, etc. In theory, the planet is in equilibrium.