Sera Myu is the abbreviated form of "Sailor Moon Musical". Since Japanese people pronounce "Sailor Moon Musical" as "See raa moon Myuuzicaru", they took the first letter of katakana of each word (underlined) and call it "Sera Myu".
But you don't have to live in Japan to see the musicals-each musical has at least one video released, and the newest musicals (starting with the 10th anniversary "Ai no Sanctuary" Musical) are going to also be released on DVD.The plots of the musicals do not always follow the anime/manga, and some plots are completely original. The music is absolutely fabulous, and the costumes are beautiful. In short, there's no reason not to love Sera Myu!