Bryan Cranston wasn't known for much before he landed the breakout role in Breaking Bad. Sure, he was Malcolm's dad, Hal, on Malcolm in the Middle, but after that his resume was pretty empty. He had some guest starring roles here and there, but his visibility in the public eye was pretty much gone.
The tattoo is super small, so it is barely noticeable, but the "Br Ba" periodic table symbol is instantly recognizable. It marks a turning point in the actor's career and shows that he is thankful for the opportunity he was offered.
This is just a testament to the show's legacy and the effect it had on the actors' lives. It was unforgettable and will live on as one of the greatest shows ever written/acted/produced.