A key digest is a string of trytes that is generated using a seed and a private key.
For multisig, each co-signer provides a key digest, and all of these key digests are used to create a multisig address.
The co-signers can safely share their key digest with other co-signers so that each one can independently validate the multisig address.
See this article for more info.
iota.multisig.getDigest(seed, index) Generates the digest value of a key.
seed: String, Tryte encoded seed index
index: Integer, Index of the public/private key pair that you want to use.
String - key digest represented in trytes