The electrons of an atom are arranged around the nucleus in shells. A shell can be thought of as a region around the nucleus.
Shells can also be envisioned as energy levels as they indicate the energy of the electrons within them.
An electron with a higher energy occupies a shell farther away from the nucleus.
It has a greater ability to resist the attractive force of the nucleus and is more easily removed than a lower energy electron.
The shell closest to the nucleus is assigned the number 1. Larger shells are assigned higher numbers according to their size rankings (e.g. the second smallest shell is assigned the number 2).
Even though electrons are sometimes drawn as dots and crosses to indicate that they come from different atoms, they are indistinguishable in a real molecule.
Each circle represents a shell (e.g. the innermost orbit represents shell number 1).