A neuron carries electrical impulses (action potentials) around the body. When the neuron is at rest (not firing an action potential) the inside has a more negative charge than the outside.
When the neuron is at rest there is constant movement of ions into and out of the cell. These ions include Chloride (Cl-), Sodium (Na+), Potassium (K+) and organic ions. The movement of these ions creates a charge across the neuron membrane called the Resting potential. In a neuron, this remains constant at about -65mV.
The balance of ions is essential for an action potential to occur. The balance of ions is maintained by two forces: An electrostatic pressure (when positive and negative ions attract) and diffusion (the movement of particles from an area of high concentration to an area of low concentration). For example, the electrostatic force causes the K+ ions to move into the cell, while diffusion causes them to leave again.
‘The Neuron at Rest’, 2016. Stack Exchange – Biology