This expression describes someone who's extremely interested in something on TV and can't stop watching it. This often happens with dramatic news stories:
After the 9/11 attacks, I remember my parents being glued to the TV for several days.
This phrase can also describe people who just watch a lot of TV:
He's glued to the TV from the time he gets home to the time he goes to bed every day.
The expression "glued to the TV" comes from the funny idea of someone being stuck to a TV set with glue. For some people who can't stop watching a TV show, it might seem like that's what happened to them.