Since the planet is a system, the only way it can exchange energy with its environment, space, is by radiation. For this reason it is important to understand the radiation budget at the top of the atmosphere. This includes incoming solar energy, solar energy reflected back to space by the earth, and terrestrial energy emitted by Earth out to space. When you balance your checkbook you are concerned with withdrawals and deposits. Deposits represent a gain to the account and withdrawals a loss.
Gains are positive and withdrawals are negative.
If you handle your money correctly, gains exceed the losses and the money in your account increases and you are happy. In energy budget studies we are also concerned with gains and losses, though we are not always seeking a positive balance. In discussing Earth's energy budget, it is useful to first consider the global radiative energy budget averaged over a year. Then investigate the energy balance as a function of latitude.