What does the plus sign over the minus sign mean?

Sometimes shown as ' +/- '

This means that the answer can be either positive(+) or negative(-).

An example is 'square rooting'.

The sqrt of '(+)4' is +/- 2

Because

+2 X +2 =(+) 4

-2 X -2 = (+)4

With the square root of negative numbers , you are moving into the realms of IMAGINARY numbers, algebraically indicated by a lower letter 'i'.

e.g.

sqrt(-4)

Becomes

sqrt(-1)X sqrt(4)

'i' X sqrt(4) = i X +/-2 = either '2i' or '-2i'.