A BitTorrent index is a list of .torrent files (usually including descriptions and other information), managed by a website, that is available for searching files to download through BitTorrent protocol. It should be differentiated from a BitTorrent tracker

BitTorrent tracker

A BitTorrent tracker is a server that assists in the communication between peers using the BitTorrent protocol. It is also, in the absence of extensions to the original protocol, the only major critical point, as clients are required to communicate with the tracker to initiate downloads...

, which merely coordinates communication between peers attempting to download the payload of the torrents. Many BitTorrent websites act as both tracker and index. Sites such as these publicize the tracker's URL and allow users to upload torrents to the index with the tracker's URL embedded in them, providing all the features necessary to initiate a download.