Think of energy levels like the rungs of a ladder. You can stand on the first rung or the second rung. You cannot stand floating in the air between two rungs.
In chemistry, we identify each main level with a principal quantum number. We use the letter n for this number, starting with n = 1. The level closest to the nucleus is labeled K (n=1).
It has the lowest energy and holds the fewest electrons. As you move outward to L (n=2) and M (n=3), the energy levels get higher and hold more electrons. A simple formula, 2n squared, tells you the maximum number of electrons for each level.
Each main level is broken down into smaller areas called subshells. These subshells are labeled with the letters s, p, d, and f. Students often confuse an energy level with a physical hard shell you can touch.
They are really just mathematical regions of probability. Electrons can jump between these different levels. To jump up to a higher level, an electron must absorb a tiny packet of light.
To drop back down, it must release a packet of light. This exact energy jump creates the unique colored light we see in atomic emission spectra.