Frequency Hopping is one of the two basic modulations used in the spread spectrum signal transmission.  It is the repeated switching of frequencies during the radio transmission.  Its is the popular technique used in military for unauthorized interception or for jamming the of telecommunications.
In frequency hopping, the transmitter “HOPS”  between available frequency channels according to the specified algorithm.  The trasmitter operates in sync with the recevier which remains tuned to the same center frequency as the transmitter.  First data is sent on the narrow band and then hops to another frequency and then transmits again.

Frequency Hopping Spread Spectrum (FHSS) is much more sophisticated then the Direct Sequence Spead Spectrum (DSSS), but FHSS is more complicated than DSSS