The action potential is an electrical phenomenon that occurs in nerve cells (or neurons). It is a self-propagating event that begins at a dendrite and travels down the axon to the end of the neuron.
The neuron starts at resting membrane potential which is around -70 mv (inside of the neuron is negative compared to outside). When the neuron is stimulated (say, by pressure), a little bit of Na+ spills into the neuron through leak channels. The entry of Na+ depolarizes the neuron and is referred to as a graded potential. If enough Na+ enters, the inside of the membrane becomes positive enough to reach threshold potential. If this happens, voltage-gated Na+ channels open allowing a large influx (entry) of Na+ into the neuron. As Na+ enters, the membrane potential becomes increasingly more positive. This causes the upstroke of the action potential.
After a millisecond or so, the voltage-gated Na+ channels close and voltage-gated K+ channels open allowing K+ to leave the neuron (efflux). As K+ leaves, the loss of positive charge causes the downstroke of the action potential. The membrane potential briefly goes more negative than normal causing a hyperpolarization. Then the Na+/K+ pump kicks in and restores resting membrane potential and the action potential is finished.
So, the action potential is called a brief reversal of membrane potential because it starts at about -70 mv, goes up to around +30 mv, and back to -70 mv. This is the nerve impulse that travels along your nerves or neurons. It can travel hundreds of metres per second!