In order to open a line of credit (credit card or loan), the lender will pull your credit record to determine your creditworthiness. The "big three" credit record companies being referenced are Transunion, Equifax, and Experian. Based on your credit "score" with one or more of those reporting agencies, the lender will approve or decline your request to borrow, and determine the interest rate they'll charge you.
"Locking your credit" means telling those reporting agencies, "Hey, don't let anyone pull my credit." Why would you want to do that? Because of identity theft. If someone gets enough information about you, they can borrow money using your identity, and never pay it back. This situation is a fucking nightmare.
By locking your credit, you gain a huge amount of control over anyone - including yourself - from borrowing from a lender. Each credit reporting agency offers this option, at no charge, and the lock can be temporarily or permanently lifted by you (login, 2FA, sometimes you can even specify a specific lender who is allowed to pull your credit while continuing to forbid anyone else).