Example: "Baseband signals must be modulated to higher frequencies for radio transmission."
A baseband signal is an original transmission signal that has not be modulated, or has been demodulated to its original frequency. Most telecommunications protocols require baseband signals to be converted, or modulated, to a higher frequency so they can be transmitted over long distances. Therefore, the original baseband, or lowpass, signals are converted during the transmission process.
When the signal arrives at the destination, it is demodulated so that the recipient receives the original baseband signal. Ethernet is an example of a protocol that does not require signal modulation, since it transmits data in baseband.