The Filecoin specification is the technical document that outlines the details of how the Filecoin network operates, including the rules for participating in the network, the format of the transactions that are used to manage data storage, and the algorithms and data structures that are used to store and retrieve data on the network. The specification is an important reference for developers who are building applications on top of Filecoin, as it provides them with the necessary information to create integrations that are compatible with the rest of network.