There are various reasons why you might have trouble understanding the dialog in TV shows or movies....sometimes there is background music that is too loud, or the character mumbles or has an accent. Also, the actors on TV and in films have memorized their lines, so they tend to speak more rapidly than non-actors do in everyday conversation. (I have a bit of hearing loss, and I've noticed that in the past few years I've avoided going to the cinema to see movies, because I sometimes have trouble understanding parts of the dialog. If I wait until the movie is released on DVD I can watch it at home and turn on the closed captions. )
When you're speaking to someone in person, you not only listen to their words you also watch their facial expressions and body language. All of these things combined help you to understand what is being said. The tone and volume of their voice also gives you clues as to what they're saying. Movie and TV actors don't really speak like "real" people when they're reciting dialog - they're "performing," not speaking, and doing so under instructions from the director.