A feature of the Radio Data System (RDS) which lets broadcasters send a 32 or 64 character message on an inaudible part of the FM channel.

This is used to send information about the current program, a phone-in number, or just general information about the station.

As an example, here's what's currently showing on my set for all the available stations.

The BBC make good use of Radiotext:
Chris Moyles: With some [Quote] "Very Special Guests"
Steve Wright in the Afternoon - Fax the show: 020 7765 0869
Jazz Legends - Julian Joseph profiles pianist Fletcher Henderson

Classic FM were changing the message frequently: I caught these:
Jamie Crick
Listen to Classic FM at the Movies at 5.20pm

Clyde 1 and Beat 106 were broadcasting their slogans:
Clyde 1 - Scotland's Biggest Hits plus Travel, News and Sport
Beat 106 - Central Scotland's best music - 105.7 & 106.1 MHz FM

96.3 QFM and Scot FM (both owned by the same people) had
Radiotext messages wil appear here

Meanwhile, BBC Radio nan Gaidheal were just plain confused:
Radiotext from                    BBC Radio Scotland