A neuron membrane potential is the build up of an electrical charge within the nerve cell. These electrical charges occur because of a build up of ions on one side of the membrane versus the other side. These ions cannot readily pass through the cell membrane at all times, hence the unequal distribution over time.
The ions present in all neural membrane potentials are the sodium cation (Na+), potassium cation (K+), and the calcium cation (Ca2+), though the concentration of the calcium ion is so small that it is negligible). So the sodium and potassium ion channels are the channels involved here. During the resting potential, no electrical impulse is being passed through the neuron, so the ion concentrations remain steady and relatively unchanging. Excitable cells can undergo either a graded potential or an action potential. An action potential suddenly opens up all of the ion channels at once and allows the ions to flow freely across the membrane at full speed. This results in a completely open and full firing of the neural signal. A graded potential, on the other hand, allows for the ion channels to open partially or at different amounts over time, thus regulating the flow of ions and the strength and duration of the neural signal. Finally, a threshold potential is the minimally open threshold of ion flow that allows for a successful nerve signal to be transmitted. You can think of it as the smallest possible graded potential.