Function calls in the Ethereum Virtual Machine are specified by the first four bytes of data sent with a transaction. These 4-byte signatures are defined as the first four bytes of the Keccak hash (SHA3) of the canonical representation of the function signature. Since this is a one-way operation, it is not possible to derive the human readable representation of the function from the 4-byte signature. This database is meant to allow mapping those 4-byte signatures back to their human readable versions.
The data from this service is given free of any license or restrictions on how it may be used.
Usage of the API is also granted with the single restriction that your usage should not disrupt the service itself. If you wish to scrape this data, feel free, but please do so with limited concurrency. You are encouraged to scrape your own copy of this data if your use case is likely to produce heavy load on the API.