A "Cool story bro" sweatshirt is a sweatshirt that has the phrase emblazoned on it, sometimes accompanied by the companion phrase, "Tell it again." It's also possible to find T-shirts with the phrase "Cool story bro" printed on them.
"Cool story bro" is an Internet meme that appears to have started initially with the 2001 film "Zoolander," in which one character responded with "Cool story" after listening to an extremely long story told by another character. The word "bro" was added to the meme in 2008 on the Internet bulletin board 4chan. Often it was paired with an image from the comic book "The Incredible Hercules." The phrase is a sarcastic remark that connotes indifference to the speaker.