In neutral state the atom got an equal number of protons (+) and electrons (-). If you lose and electron then you will have 1 more proton than electron, so you have a +, but if you gained an electron, then you'd have one more electron than protons, so you'd get a -.
Fx: carbon has 6 protons and 6 electrons. If carbon loses an electron, then the carbon atom would have 6 protons and only 5 electrons, then you'd have 6+ and 5-, which is 1 more plus and minuses, so you have C+.
If Carbon gained an electron, then you'd have 6+ and 7-, which is 1 more minus than pluses, so you'd have C-.
Last example, if you gained 2 electrons, then you'd have 6+ and 8-, which is 2 more minuses than pluses, which would give you an C-- (Also known as: C2-)