Short for modulator/demodulator, it is a two-way communications interface working between a communications channel, such as a DSL line, and a machine such as a computer. That is how billions of people access the internet. Television itself is distributed live via a modulator in the transmission chain, and a demodulator at the receiving end.
The efficiency of modern modems over analog is worth noting. In analog days it generally used PAL or NTSC ‘coding’, that compressed three channels R, G and B into one, which was transmitted in one TV channel. Today there are very efficient compression systems such as MPEG-2, MPEG-4, H.264 and H.265 used in DVB, ATSC or other digital systems. The combination of the latest video compression, H.265, and the efficiency of DVB-T2 modulation will allow up to 32 SDTV channels, or 4 HD channels, or one 4K UHD channel, to be delivered in the space (bandwidth) that was occupied by one analog PAL 625-line channel.