There are 3 major credit reporting bureaus in the US which take reports on every American’s credit worthiness. Every time an American makes a payment, that information makes it to the bureaus. And before a new account is opened, a lender will ask the bureaus for a copy of that person’s report.
When someone locks their credit file, they instruct the credit bureaus to not send those credit reports to lenders, which will prevent the lender from issuing the loan. They do it when they know that they will not apply for a loan, be cause it prevents fraudulent loans from being opened in their name.
A common practice of identity thieves here is to open up a bunch of new credit cards or other loans in other people’s names, and run up the charges before that person finds out about the account and has it closed.
Chexsystems is a special case, I know them as a clearinghouse to report people who write bad checks. But people are using checks less and less these days.