What does the outer electronic layer mean?

Electrons can be in an atom only at certain electronic levels (orbitals). The orbital that is farthest from the nucleus is called the outer electron level. To determine the number of electrons on the outer (last) level, you need to know a) the number of the level is equal to the number of the period in which the element is located; b) for elements of the main subgroups, the number of external electrons is equal to the number of the group; c) for elements of side subgroups, the number of external electrons does not exceed two.