What is radio frequency receiver?

The definition of what it is, is actually in your question. It's a receiver that operates in the radio frequency spectrum. Anything that has a transmitter is said to transmit in radio frequency or `RF`. The term radio is short for radiotelegraphy, which has become generic over the years.

Where does the radio frequency start? Technically, it' at 10,000 hz and higher. That's not to say that there's lower frequency transmitters (The U.S. Gov't uses 2 hz. to reach our submarines when they're under water, but that's more of an exception than the rule. The radio frequency spectrum can extend into the gigahertz range for things such as satellite communications, and as our technology advances, we could eventually see frequencies in the teraherts.

So a radio frequency receiver is anything that will receive a a frequency in the radio frequency range.

Hope this helps,