The neuron's resting voltage

Every cell has a voltage difference between what is inside and what is outside, separated by the membrane that forms its boundary. When the cell is quiet, not excited, that difference is called the resting membrane potential.

What keeps it there is a pump: the sodium-potassium pump, which sends three sodium ions out and brings two potassium ions in. That is how it creates the concentration gradients across the membrane. And at rest potassium is the ion that crosses the membrane most easily, so it is the one that ends up setting the voltage.

Why the inside stays negative is easiest to picture with a bank account. As those positive charges, the cations, leave, negative charge is left behind: the inside account goes into the red and the outside one grows more positive.

This matters because some cells, neurons and muscle cells, are excitable: they can come out of rest and shift into an excited state. That resting state is precisely the starting point they take off from.