The symbols on this t-shirt for Potterheads are a number in Ancient Runes : 62442. And 62442 is the number that enable you to enter the Ministry of Magic through the visitor’s entrance (in Harry Potter and the Order of the Phoenix). If you dial 62442 on a phone, it writes MAGIC. =)
J.K. Rowling created these Ancient Runes numbers. You could find two pages from ‘Ancient Runes Made Easy’ that explained which symbol stood for which number and why in her previous official website.The numbers are represented by fantastic beasts (here a Salamander, two Graphorns and two Fwoopers).