A talkgroup will use a frequency to transmit/receive. Then another talkgroup could use this same frequency for another transmission.
When a trunked radio requests to speak (pressing the transmit button) it is sending a signal to the trunked system controller saying "Hey, I need to talk to everyone in my group, please assign us a frequency to use". Then the system controller moves all the radios on that group to an available voice channel.
The talkgroup ID is a way of identifying a given group of users.