Why does the sodium ion have at 1 charge?

When an element has a + or - next to it,

this refers to the element in it's ionic or charged form.

Each element has a set number of protons neutrons and electrons, but most elements ,like sodium -are not in their most stable form unless they complete their electron valence shells-

This can occur by either giving up electrons or grabbing onto electrons.

Sodium has 11 electrons

2 in its its first orbital (1s2)

6 in the second(2p6)

and 1 in the outer orbital (2s1)

But Sodium is not stable with one lone electron spinning around,

so it will always give it up whenever possible, hence it will always be one electron short- giving the element a positive charge.