electric potential

Electric potential is formally defined using "work done". As you have said, it is defined as the work done required in moving a small positive unit charge (a +ve test charge) from infinity (which is of potential zero) to the point of interest.

A negative potential (potential < 0 volt) simply means work needs to be done in bringing a +ve test charge from the point to infinity. Likwise, a point where the potential is zero means that the potential at that point is the same as that at infinity. It is clearly no (net) work needs to be done on it when bring it to infinity.

These definition are mathematical. In the actual fact, the absolute potential carries not much physical significance. Just in analogy to gravity (gravitational potential), there is not much difference to your feeling if you stay at the ground floor (analogy to zero potential) or the fifth floor of a building (a +ve potential), or in the basement (analogy to -ve potential). It is only a difference in potential that has physical significance. Just like it really matters if you fall from the first floor of a building compared with when you fall from the tenth floor of the building.

有2個charges , 一個+q , 一個-3q。它們相距1米。
V=0有邊幾點?
You may refer to the following web-page on which you could imput the amount of charges in the animation. The zero potential line (the line that separates the red and blue colors) will be clearly seen.

http://lectureonline.cl.msu.edu/~mmp/kap18/eField.htm