baseband transmission

When you write a message on a long strip of paper and slide the whole strip down a tube, the message uses the entire tube and nothing else can share it at the same moment. That is baseband transmission: the digital signal is placed directly on the medium, occupying the full bandwidth of the channel as a single stream, with no carrier wave and no other signals sharing the wire at the same time. The bits are sent as they are (after line coding), straight onto the wire.

Its opposite is broadband transmission, where signals are modulated onto carrier waves at different frequencies so that many separate streams can travel down one medium simultaneously, each in its own frequency band — much like many radio stations sharing the air. So 'baseband' means one signal using the whole channel near zero frequency; 'broadband' means many modulated signals carved up across the frequency spectrum. In everyday marketing 'broadband' has drifted to just mean 'fast internet', but technically it is about sharing one medium by frequency.

Most local-area networking is baseband: classic and modern wired Ethernet send a single digital signal down the cable using the whole bandwidth, which is simple and efficient when the link is point-to-point or short. Cable television (and cable internet) is the textbook broadband case: one coaxial cable carries dozens of TV channels plus your internet data, each modulated onto its own frequency band, all at once. The choice is about whether you dedicate the medium to one stream or slice its frequencies among many.