When neurons are not producing electrical signals we say that they are at resting phase: voltage across their membrane is called the resting membrane potential, or the resting potential.
This potential is determined by the concentration of ions (Na, K) across the membrane and by membrane permeability to each type of ion. While the ions move through channels down their gradients they lead to a separation of charge and that is what creates the resting potential.
The membrane of the neuron is much more permeable for K ions so the resting potential is close to the equilibrium potential of K+.
