FastText was (is) an enhancement of the Teletext system available in most TV sets in the UK.
Briefly (as there's a whole node with more information on it), Teletext comprises of hundreds of pages of textual information, transmitted on a few spare lines of the normal TV broadcast. Any TV with a teletext adapter can decode these and display them. Pages are selected with a numeric keypad on the remote control.
FastText TVs provided an extra function. The remote control had four additional buttons across the bottom, coloured red, green, blue and yellow. On every page you went to, there were corresponding red, green, blue and yellow notes at the bottom of the page corresponding to the page you were most likely to want to go to next. For example, the "News Headlines" page might list "World News", "UK News", "News Summary" and "Recent Stories". Or the TV listings page would list four channels across the bottom.
Therefore, rather than dialing in the page number for the page you want, you could simply press the coloured button on the remote control and you would go straight there. The links weren't customisable - they were specified in the broadcast. But of course, every page would have its own FastText links.
A further enhancement reduced not only the effort of typing in the page number, but the delay in waiting for it to appear. Teletext transmitted all the pages in quick sequence, so it can take up to 30 seconds sometimes for a new page to appear. Many FastText TVs, on loading up a page, check what the FastText pages are, and then pre-load them as soon as they come round. This means that they appear immediately on pressing the coloured button.
One of the cleverest other uses of FastText was to provide a semi-interactive quiz system. Officially, TeleText only supported numeric pages. However, pages could also be transmitted with "page numbers" that actually included letters. There was no way to dial in these page numbers on the remote control. However, the FastText buttons could be made to link to them.
So, the first page of the quiz would come up, with four multiple choice answers, and these would all be represented by the FastText buttons. Pressing the correct one would take you to a page with the next question (and four answers again), pressing any of the others would take you back to the start.