It was essentially like having a text-only version of the web built into your TV, but back in the 80s. TV channels would broadcast the data for the pages in the gaps between video frames and a TV with the appropriate decoder would cache the data and use it to build up the complete set of pages, which you would then access by pressing the "text" button on your remote control. Each TV channel would broadcast its own Teletext service, with most channels offering a mixture of news, sport, entertainment etc.
It sounds (and looks) primitive by modern standards, but it was actually a wonderful technology that worked perfectly well for its intended purpose and was very information-dense. Sadly, we lost Teletext in the UK during the changeover from analogue to digital TV, but I think it's still alive in a number of countries.