"Show don't tell" usually refers to action lines. With dialogue people usually refer to it being to "on-the-nose." In my opinion it is more or less the same issue.

There is a lot of information to find on this topic all over the internet you just gotta look for it.

For an example:

Say character X just heard that his grandma had a heart attack and is at the hospital.

Telling would be something like "X is sad to hear about his grandma". You just literally told the reader what X feels but how does this feeling translate to what you can see on the screen?

Showing would be more like "X's eyes fill with tears, he sinks to the ground and buries his face in his hands.". This is what the audience would see watching the movie. Seeing that (or reading it in the script for that matter) we know that the character is sad, we don't need to be told.