ok...this is a little tricky, if you want you can read up on ions, compounds, and oxidation.
Hg2O is a neutral compound (charge = 0) made up of :
->the positive ion Hg1+ (1+ is a superscript, it's its "charge". it has different oxidation states, the most common of which are Hg1+ which is your Mercury(I) and Hg2+ which is your Mercury(II) )
->and the negative ion O2- (the 2-, its charge,is also a superscript)
when you combine these two to make a compound it takes two of the positive Hg1+ to balance out the charge of the O2-; hence the final neutral product Hg2O (with the 2 as a subscript, durn YIA not having subs ans supers, grr)
you basically have this balanced equation:
2(Hg1+) + O2- --> Hg2O
look at the charges:
2(1) + (-2) = 0
2 - 2 = 0
does that help?
you can email me or something if it's still confusing.