What is a voltage-gated membrane channel?

Voltage-gated channels are proteins in the cell membrane which open when stimulated by a voltage (an electrical signal). The voltage causes the channel to open, thereby allowing the entry or exit of whatever substance the channel relates to.

An example of this the the voltage-gated sodium channels on neurons. When an action potential (a voltage), passes over the cell, it open these channels and allows sodium to enter the cell.