The internal composition of a neuron is markedly different from that of the fluid outside of it. This potential difference, in the absence of external stimuli, is called the resting potential.
Different neurons have slightly different resting potentials.
If there are more positive charges inside than outside, we say it has a positive membrane potential. Either way, we assume the outside to be zero. In electrical terms, we would say the extracellular fluid is grounded. At rest, sodium is much more concentrated outside the neuron than inside. As explained earlier, the internal composition of the cell is more negative than the fluid outside.