OP_CHECKDATASIG and OP_CHECKDATASIGVERIFY check whether a signature is valid with respect to a message and and a public key. The message is restricted to exactly 256 bits, if longer messages are to be signed, it is recommended to sign it's hash.
This permits data to be imported into a script, and have its validity checked against some signing authority such as an "Oracle".
OP_CHECKDATASIG and OP_CHECKDATASIGVERIFY are designed to be implemented similarly to OP_CHECKSIG [1]. Conceptually, one could imagine OP_CHECKSIG functionality being replaced by OP_CHECKDATASIG, along with a separate Op Code to create a hash from the transaction based on the SigHash algorithm.