KYC (know your customer) is the operator’s legal duty to establish who the player is: that they are of age, that they open the account in their own name and that the money does not come from illegal sources.
For an affiliate KYC has two effects. First, some sign ups never pass the check and never become qualified players. Second, slow verification lowers conversion to a first deposit. When comparing programmes, ask how long the check takes and whether CPA is counted before or after it.