Shells and energy levels
The orbits the electrons move in belong to shells around the nucleus.
Counting outwards from the nucleus, the shells are named 1, 2, 3, 4 … or, equivalently, K, L, M, N … — two names for the same thing, and you will meet both.
Figure to be added
Shells are energy levels
These shells are also called energy levels, and the reason is that each one carries a fixed amount of energy. An electron in a given shell has the energy of that shell.
Two things happen as you move outwards
The energy rises. Going outwards from the nucleus, each level holds more energy than the one inside it.
The gaps shrink. The difference in energy between neighbouring levels gets smaller the further out you go.
Those two facts pull in opposite directions, and it is worth holding both. The levels climb — but they crowd together as they climb, so the steps get shorter the further from the nucleus you are.