Know Your Customer (KYC) it is a part of AML (anti money laundering) efforts implemented at online casinos. Online casinos that offer real money gambling will inevitably have to deal with financial transactions. Some casinos can turn over hundreds of thousands or even millions of dollars in a single day. This makes them susceptible to money laundering and other types of financial crimes. Furthermore, since all gambling happens online and there is virtually no face-to-face interaction, it is even easier for fraudulent users to target online casinos. KYC verification procedures are thus introduced in an effort to curb such fraudulent activity by verifying the identity, age and address of every user.
Licensed online casinos are legally required to complete KYC verification of their customer accounts before processing any withdrawals to the account and this is why the casino usually requests KYC documents prior to your first withdrawal. Even for winnings made with no deposit free spins, the same regulatory mandates will apply before you can cash out. In order to complete the KYC verification, you will generally have to send copies of documents proving their identity and address.
Some commonly accepted documents include passport, national ID, drivers license, a recent utility bill, etc.