An action potential is the firing of an electrical impulse along a neuron. A neuron is a nerve cell that consists of the soma, dendrites, and an axon. The electrical nerve impulses that a nerve cell conducts travels along the axon, through the dendrites, and to another neuron. This is how nerve impulses travel from one part of the body to another.
When not firing, a neuron has a net negative charge inside the cell versus the outside. Various ions like sodium are not able to easily pass through the cell to balance out the charge. Just prior to firing, the electrical potential of the cell increases and sodium ions are able to pass through the cell membrane to the interior of the cell to balance out the charge. After this occurs, the electrical signal passes through the cell, an event called the action potential. After it passes, the neuron goes back to resting state and the sodium ion charge builds up again.
Action potentials are often referred to as all-or-none. That is to say, there is no variation to the amplitude of the signal. In other words, a neuron cannot fire weakly or strongly. The electrical signal passes through the cell at full speed until it is complete. It is either fully open or fully off. Since the electrical signal passes through at the speed of electricity, there is really nothing that can change the speed of the action potential. In terms of the frequency of the firing, this depends on how often the nervous system sends an impulse.