Trunking is a new world. Don't worry, you'll understand it with time.
In a nutshell, you have a handful of frequencies, with many agencies that operate on them.
In a conventional system, you're used to your Police, EMS, Fire, Etc., being on their own special frequency. In a trunked system, it's not like that. Instead of frequencies, each agency, such as EMS, has a talkgroup. The talkgroup is a special code that is assigned to all the radio's in each ambulance. The Police have their own talkgroup as well, which is assigned to all the radio's in each patrol car, and to the walkie talkies the officers carry.
At any given time, the Police could be on one frequency, and 3 seconds later, they could be on a completely DIFFERENT frequency. This is accomplished by a central computer, that identifies the talkgroup, assigns it a frequency, and "tunes" the radio's that need to be recieving the talkgroup, so they can hear it.
It's a much better way to have several agencies on just a few frequencies. In a big city, you could have several hundred talkgroups, including Police, EMS, Fire, all the way up to trash trucks, Water companies, Security guards, and more, using only 13 or 15 frequencies.
In a conventioanl system, several hundred agencies would need several hundred frequencies to accomodate them all, and here, they all SHARE a set of frequencies. As microphones are keyed and unkeyed, the frequencies are recycled. So if you monitor one frequency, you could hear the Police there one minute, and the next minute hear the EMS going on a run. Trunking scanners hold on an individual talkgroup, instead of the frequency.
You'll learn!