Every living cell's membrane actively maintains a stored electrical voltage across itself at all times, called the resting membrane potential, with the cell's interior held at a negative charge relative to the fluid outside it. This isn't a passive, static feature; it's continuously maintained through active work by the cell, and in excitable cells like neurons and muscle cells, an action potential is simply that carefully maintained resting voltage being briefly, dramatically discharged and then actively restored, a rapid electrical event built entirely on top of this baseline resting state.
The resting potential is actively maintained, not simply a default state
A cell's resting membrane potential exists because of an uneven distribution of charged ions, sodium, potassium and others, across the cell membrane, maintained specifically by membrane proteins including the sodium-potassium pump, which actively transports ions against their natural concentration gradients using cellular energy. Maintaining this uneven ion distribution, and the resulting voltage difference it produces, requires continuous energy expenditure by the cell; if that active maintenance stopped, the resting potential would simply collapse as ions redistributed themselves evenly across the membrane, which is exactly why the resting state is accurately described as actively maintained rather than a passive default the cell simply settles into.
An action potential briefly spends and then rebuilds that stored voltage
An action potential begins when specific ion channels in an excitable cell's membrane open in response to a strong enough stimulus, allowing ions to rapidly flow across the membrane and briefly collapse and then reverse the resting voltage difference, before other channels and the sodium-potassium pump work together to actively restore the original resting state afterward. This entire rapid sequence, discharge followed by active restoration, only works because the resting membrane potential was already carefully, energetically maintained beforehand; without that pre-existing stored voltage to discharge, there would be nothing for an action potential to actually spend.
What we're still unsure about
The basic mechanics of the resting membrane potential, and its active maintenance through ion pumps and channels, are precisely characterised, extremely well established cell physiology, confirmed across an enormous body of experimental research. What remains more genuinely an active area of ongoing research is understanding exactly how membrane potential and its regulation vary across the full diversity of cell types beyond the well-studied excitable cells, since many non-excitable cells also maintain and actively use resting membrane potentials for purposes still being actively characterised, rather than every cell type's membrane physiology being as thoroughly mapped as neurons and muscle cells already are.
This sits inside Membrane Physiology & Action Potentials, one of seven topics in Physiology, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.