Equilibrium potential is the difference in charge relative to the inside of the cell. -90 mV means that the inside of a cell is more negative than the outside. However when you talk about equilibrium potentials, I thought you only consider an individual ion like potassium? Potassium usually has 150 nM inside the cell and 5 nM out. If you allow potassium to freely pass through the membrane, wouldn't it eventually balance out so that the net charge on both sides is 0 (concentration would also be equal)? So why do people say that the equilibrium potential for K+ is -90?
Most ions don't pass freely through the membrane. This is prevented by the hydrophobic (water "fearing") fatty acid tails of the phosholipids that provide the basis of the membrane. The regions outside and inside the membrane are aqueous and attract ions much more than does the hydrophobic fatty region in the interior of the membrane bilayer.
Under usual physilogical conditions, the only way ions can cross the membrane is through protein channels. But these channels are gated and only open in response to certain stimuli. Stimuli can include a chemical substance binding to the gate and opening it, this is how neurotransmitters work. Many gates respond to changes in the overall potential across the membrane. These are termed voltage regulated gates and, among other finctions, explain how action potentials (impulses) travel down neurons and muscle cells.
Regardless of how these gates open, the effect is temporary. Neurotransmitters break down, the membrane potential continues to change in a manner that voltage regulated gates once again close.
During these events only small amounts of ions cross the membrane compared to the vast supplies on whichever side has the surplus. That would be the interior of the cell in the case of potassium. Yes, this might eventually deplete the concentration if allowed to continue without some mediation.
That mediation is the sodium-potassium pump. The Na-K pump, using the energy in ATP, pumps Na+ out and K+ into the cell. But it pumps out 3 Na+ for every 2 K+ it brings in which makes the overall resting membrane potential -70 mV. This pump is busy and important. It is responsible for using about 20% of the ATP your body produces. Even more if you're inactive.
So, after some of the K+ ions are allowed out through gated channels, the potential changes very locally. Then other K+ ions migrate into this area, down their concentration gradient, restoring mmost (but not all) of the normal K potential. Operation of the Na-K pump provides the additional K+ ions to restore it completely