How Neurons Work

A neuron is an excitable cell. They carry electrical charge when stimulated.  When a cell gets excited, Na+ gates open in the cell membrane to allow sodium ions travel across the cell membrane.  These sodium ions are positively charged. A cell that is more positive than resting is known as Depolarization. Depolarization causes changes of the permeability cell membrane of any excitable cell. K+ is also positive in which leaves the cell membrane, taking its positive charges with it. The inside cell becomes more negative again returning the membrane potential toward resting aka repolarization.  

A cell that is not stimulated or excited is called a resting cell and is said to be polarized. Polarized means a cell that has a difference in charge across its membrane so it is more negative on the inside than on the outside of the membrane.