Language is always evolving. New words are entered into the dictionary every year, and many of those words are invented by young people. There will have been slang terms that you used when you were younger which now sound dated and ancient to people ten years younger than you. They'll have their own language, which is full of expressions like 'swag' and 'YOLO.' The terms that the young use can sound like a foreign tongue to the uninitiated, but this man seems to be on board with it.
To a young person, nothing will make a term seem less cool than seeing their dad wearing it on a t-shirt. He might even have borrowed this shirt from his child for the sake of this picture. Either way, we're sure they'll never use the expressions again! If you're not sure, by the way, 'YOLO' is an acronym for 'You Only Live Once.'