This is a term used to refer to someone that's observed or analysed something that has already been implied or is obvious.
Commentator of a game: Mumbai Indians lost the match due to their poor batting display.
David: Mumbai could've batted better.
David's friend: Why does this guy give his tube-light analysis all the time
Someone who takes a long time to get something...i.e. he/she is slow. The word's short form is tubie...it literally means a florescent bulb... because when you switch it on the light doesnt actually flash until a few seconds later. Similarly, the person's brain whom you called a tubelight, doesn't light up and understand something until later
Rose is such a tubelight, she didn't get the joke until after everyone stopped laughing