In terms of the black zig–zag line:

(i) It 'roughly' divides the metals on the left from the non–metals on the right of the elements of the Periodic Table - some elements by the zig-zag line can have a mix of metallic and non-metallic properties - referred to as semimetals or metalloids e.g. Boron B, silicon Si, germanium Ge, arsenic As and antimony Sb.

(ii) Elements on the left form giant metallic lattices - metals - in alloy combinations the metal atoms bond in the same way as in the pure elements

(iii) Non-metallic elements on the right combine with each other with covalent bonds to form molecules.

(iv) When elements from the left and right combine, the bonding maybe covalent or ionic

BUT, when a group 1/2 metal with just 1/2 electrons in the outer shell,

combines with a group 6/7 non-metal with a nearly complete outer shell,

the result is usually an ionic compound (see the electron configurations below).

The electronic structures of the first 20 elements of the Periodic Table from 1 to 2.8.8.2

(more advanced students need to know the s p d and f notation)

You need to know about these to understand the details of chemical bonding for an element whether it is covalent or metallic or if a compound has ionic or covalent bonding.