IndexTank provides a real-time search service that allows developers to add powerful search capabilities to their applications without having to implement it themselves. It allows for features like real-time indexing and searching, custom scoring, autocomplete, faceting, geo search, and advanced text search. The speaker discusses how he used IndexTank to build search features for two sample applications in a short period of time with little effort. He also highlights reddit.com as a case study of a large site that uses IndexTank to improve its search capabilities.