Nerds and early fans of The Simpsons are most likely to get the joke behind this t-shirt design.
For the rest of you though, let me explain. When you expand the equation rdr2 you get rdrr, which when read out loud sounds like Har Dee Ha Ha.
Apart from that humour, I tried to make it so that it looked like a modern and stylish clothing brand so that it wouldn’t look too nerdy. The hope was that people would think it was a new brand, and when they asked me what it means I would giggle to myself.