Baseband communication is unmodulated communication using a simple transmitter and a stream of information that uses all of the available communications capacity. Baseband transmitter systems transmit a single signal. The signal could be electrical, radio frequency or light. The signal is not modulated--however some other device may accept this commuinication as input and create a second modulated signal from this baseband signal.
The usage of the term baseband varies slightly based on where it is used. Radio frequency communication (microwave especially), this is often the base unmixed frequency used for transmission within the facility and before it is modulated onto a carrier wave and transmitted to another station or satellite.
Computers use 'baseband' to mean a single signal is being transmitted on the wire.
Compare and contrast the meaning of baseband with broadband--note that baseband and broadband are not opposits and one is not necessarilly 'faster' than the other.