I assumed that you understand about the meaning of energy level of electrons in atom.
The electron is likely to be in the lower energy level (ground state).
It might be excited to higher energy level with external source.
Normallly, there are higher probability to find electron in lower energy state.
When an electron are excited to higher energy state, it will fall back to lower energy state in a chort time (around 10[sup]-6[/sup]-10[sup]-8[/sup] s)
The are selection rule between different energy levels. If it satisfy the selection rule, the probability to jump from one state to another state is much higher,otherwise it become very difficult.
In the case of two energy levels state:
If it is easy to jump from lower energy level A to higher energy level B, it is also easy to jump back from B to A.
If it is difficult to jump from lower energy level A to higher energy level B, it is also difficult to jump back from B to A.
And the probability to find electron in level A is always higher than B,
where is the energy difference between those two states.
In the case of three energy levels state: A , B, C
It might be that it is easy to jump from A to B (the highest energy level), and it is also easy to jump from B back to C
However, it is difficult to jump from C back to A (because some selection rule is not satisfied so the probability to make transition is smaller than normal case)
Then, it is possible to pump electron from A to B and later find out more electron in energy level C than in energy level A.