preResolve hook is called before the Operation gets resolved. As it's not a "mutating" hook, you cannot use it to modify the input of the Operation.
The main purpose of this hook is e.g. logging.
user: The user object when the user is authenticated
clientRequest: The original client request object, including Headers
log: The logger object
internalClient: The internal client object
response: The response object (only for postResolve hooks)
input: The input object (only for Operation hooks)
internalClient, you're able to securely call into all defined Operations, e.g. to talk to a database or another service to enrich a response or manipulate the inputs of an Operation.