Chronicle is the permanode (sometimes also called indexer or scanner) for the IOTA-based networks. It connects to a Hornet via the IOTA Node Extension (INX) interface. Through the INX interface, Chronicle listens to all blocks in the Tangle that are referenced by a milestone and stores them in a MongoDB database.
Chronicle offers several APIs that are documented here.
The documentation for Chronicle can be found in the IOTA wiki: https://wiki.iota.org/chronicle/welcome