This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
D23 is the official Disney fan club established by the Walt Disney Company in 2009. The name refers to “Disney” and the year the company was founded (1923). Membership provides access to content about the Disney Parks and Walt Disney Animation Studios as well as fan events, experiences (like studio tours), discounts and merchandise. There is a free general membership and a paid “gold” level membership that offers wider benefits. “D23: The Ultimate Disney Fan Event,” is a large exposition event for fans—formerly “D23 Expo”—occurring in Anaheim every other year with extensive exhibits, presentations, panels, contests and merchandise.