Resting membrane potential refers to the neuron that has a voltage across its membrane. It is determined by the concentration gradients of ions that cross the membrane and by membrane permeability to each type of ion.
For example, when the resting state happens, the sodium-potassium pump maintains the difference of charges across the cell membrane. It uses energy in ATP in order to pump positive sodium ions (Na+) outside the cell and potassium ions (K+) into the cell. The inside of the neuron becomes negatively charged compared to its outside environment as a result.