The input frequency is the frequency that all of the mobile and portable units transmit on. They transmit on one frequency (the input) and it goes into a device called a REPEATER which is a device attached to an antenna very high up, usually on top of a tower much like the ones you see all over when driving, and it rebroadcasts what the unit is saying on an OUTPUT frequency.
So everyone LISTENS on 460.5000 mhz, but when they transmit, they transmit on 465.5000 mhz. Since you're only listening (a scanner) all you need to program in is the output. And you'll hear everyone who is talking through the repeater. We (meaning RadioReference) provide a column for the "input" frequency just in case the agency is using a repeater, and its always nice to know what freq they transmit on...
as a side note, if you want, you can program the input into a separate memory channel on your scanner and scan it, so if you begin to hear someone talking on it, its a good bet that they're very close to you. Because usually when a mobile or portable transmits you'll only hear them within feet to 6 miles or so from your location. But monitoring the output will allow you to hear everyone for several miles, 20-30 miles sometimes even further, depending on how high the antenna is and the repeaters power output.
Some times I can sit at home on the coast and hear Alexandia PD in alexandria LA. that's like a 3 hour drive from me, so well over 100 miles and I can usually pick up their repeater, but would never hear anything on their input freq unless i'm within a couple of miles of the unit talking. I hope this cleared it up a bit for you.
