In terns of the database listings, "frequency" is what you want to receive. If you were transmitting "input" would refer to the transmit frequency in a repeater system. For monitoring, the output of the repeater is what you generally want to listen to. Listening on the input is useful sometimes to determine how near the station or unit is to your location.
In systems that don't use a repeater there is no input.
Rarely, systems like the California Highway Patrol, use separate input and output frequencies, but no repeater. In such a case you have to program both frequencies to hear both sides of the conversation. You will only hear nearby mobiles.
Frequency is the radio frequency and is usually measured in Kilo (1,000) hertz or Mega (1,000,000) hertz. Input means the frequency you transmit to the repeater and the input frequency to the repeater receiver. Transmit is the frequency you send to the repeater (or to another station in simplex) or the frequency the repeater sends to you.