usually that's just a video pattern, like a slide, that is static and fills the channel. For example, the color bar test card. Some will have some text indicating who owns it. Sometimes the audio carries a tone, sometimes it's silent.
They can be used to test equipment, but also to "mark their territory", so that we can tune the receivers even at times where they are not broadcasting.
Many test cards have very specific patterns, and allow a trained technician to quickly measure many indicators of transmission quality just by observing specific details
Back in the early OTA TV days we called them a "Test Pattern". They looked a little different than what you see today but they did have the different colors and sometimes a station ID. Back then TV stations didn't broadcast programming 24hrs a day.
Test card is British English. We never called them test patterns. That's probably where Lyngsat got the term from.
See my profile picture for a long-running British classic.
Test card trivia: The "x" on the blackboard on the BBC's Testcard F is at the exact center of the frame, enabling TV engineers to line up TVs. Yes, that was all done in the days of analog TV and cathode ray tubes.