Sodium is an atom represented as na+. what does the "+" represent?

Sodium - Na  is a neutral atom with 11 protons and 11 electrons.
protons are positively charged and electrons are negatively charged. Since the number of protons and electrons are equal, there are equal amounts of both positive and negative charges therefore they cancel out giving Na an overall neutral charge.
Na has 1 electron in the outermost energy shell, to become stable and to gain a noble gas configuration it loses the electron in its outermost shell.
Once it loses its electron, there are 11 protons and 10 electrons, having an overall positive charge of +1.  Once atoms lose or gain electrons they become ions, since Na loses electrons its called a cation.

So in Na⁺, +1 stands for the net charge of the ion which is +1 or in other words Na has just lost one electron therefore is +1 positively charged

Rn in the right answer