A channel has multiple definitions depending on the context. In VoIP, it refers to a unit of measurement. This unit denotes something that is either in-use or not but represents one part of the maximum number of simultaneously in-use items.
A channel in trunking may represent the number of calls that can be handled by the system at the same time.
A channel in an IVR may represent the number of calls that can navigate the IVR simultaneously.
As an analogy, picture a house with five rooms. Each room has a lamp, connected to a light switch. Each lamp is either on, or off. Now imagine that only 3 lamps can be on at the same time. To turn on a fourth lamp, you have to turn one that is on already. In this case there are three channels.