Answer
It means that in a neuron at rest-- not electrically stimulated--the inside of the neuronal cell membrane is electrically negative with respect to the outside .
Work Step by Step
This means that the inside of the neuronal membrane is electrically negative with respect to to the outside when the neuron is at rest --not stimulated. The resting membrane potential of a neuron is -70to -75 mV. Threshold is about -55 MV . The neuron is hyperpolarized if after the action potential is triggered and the Na+ channels close, continued efflux of K+ ion increases the negativity past -75 mV to -85mV or -90 mV.
