Voltage is like air or water pressure, and there is an ambient pressure in a system (like the atmospheric pressure) that we arbitrarily call zero. Negative () voltage would mean that there is enough voltage/pressure to force electrons to be emitted to ground (ie the ambient pressure) and positive () voltage would mean that electrons can be accepted by ground.

*) Negative and positive are a bit confusing because we originally assumed the force carrying particle was positive, but the electron turned out to be negative; there were some historical quirks as a consequence and some things are backwards from what you would anticipate, and sometimes I get them wrong.