From the site introduction:
Improve your characters and campaigns with content, tools, articles, and applications you'll find here at Dungeons & Dragons Insider. Your subscription to D&D Insider includes:
Dragon magazine and Dungeon magazine online
D&D Character Builder
D&D Compendium
D&D Adventure Tools
Art and Map Galleries
User summary:
D&D Insider, or DDI, was announced in 2007 as an electronic supplement including a "virtual dungeon", an online character builder, and a compendium of online resources for the game. The original scope of the product continued to change through abt. 2014 when Dragon and Dungeon were discontinued. Since abt. 2017, active development on DDI has stopped. DDI has generally been replaced by D&D Beyond.