This is the component of signal that is modulated. It is used to separate signals so that more than one message can be transmitted in a medium.

For example: The air (our medium) has many signals in it at all times. They all travel on different carrier frequencies like 900 MHz for your cordless phone or any frequency that you listen to on your FM radio in an automobile. Each station has its own carrier frequency. Another medium could be copper wires, this is how DSL works. Voice comes in at the base frequency and the DSL comes over the wire with a higher carrier frequency.