This is the Flash, Jay Garrick, t-shirt.  Jay Garrick is the first to use the name Flash. Jason Peter Garrick was a nerdy college student prior to 1940 who accidentally inhales hard water vapors after falling asleep in his laboratory. Oops. This story was eventually changed to Jay inhaling heavy water vapors.  As a result, Jay finds that he can run at superhuman speed and now has super fast reflexes. After a brief career as a college football star, Jay sports a red shirt with a lightning bolt and a stylized metal helmet with wings (based on images of the Greek god Hermes, or copycatted Roman {and more popular} god Mercury.   ... so anyway, Jay begins to fight crime as the Flash. The helmet belonged to Jay's father, Joseph, who fought during World War I. 

Jay's first case involved battling the Faultless Four, a group of blackmailers who were inept at coming up with supervillain names. In the early stories, it seems to be widely known that Garrick was the Flash. It is later explained that Jay keeps his identity secret without a mask by continually vibrating his body while in public so that any photograph of his face would be blurred.  You can blame this stupid stunt on the highly imaginative DC writers. 
The art work on this Flash shirt is done by the amazing Alex Ross.  Nobody knows how long the manufacturers will keep making this shirt.