Your credit, which is a fancy name for the profile that financial companies build on you to determine whether you are able to pay back loans. When you apply for a credit card, get a loan, rent an apartment, or buy a car, the seller will look at your credit to determine whether you are a risk of not paying and will use this info to set interest rates and payment plans.

Locking your credit means preventing these financial institutions from releasing your financial information to people who request it. This will prevent malicious actors from opening lines of credit in your name, but it will also prevent you from doing so as well.

Unfortunately in the US we can't tell a single entity that we do not want this information released. We need to inform multiple entities not to release this info since they are all independent.

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).

It is highly recommended to lock your credit. Frankly, it should be locked by default. In September of 2017, Equifax announced a data breach that exposed the personal information of 147 million people. That's essentially everyone in America who can take out a loan.

OP's post indicates that there are at least two lesser-known reporting agencies that you should look into to secure your credit reports.

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.

Locking your credit means that banks and other companies can't do a "hard pull" (meaning get a full copy) of your credit report, which is done when opening a new line of credit, like for credit cards, loans, mortgages, etc.

When it's locked, even if someone steals all your personal information, they won't be able to open a loan in your name. The best practice is to always keep it locked unless you're applying for credit.

There's three major credit agencies in the USA: Equifax, Experian, and TransUnion. This post is taking about the smaller agencies.

I think they’re referring to a credit freeze for when your identity is stolen

This does apply in other countries, like the UK. But not all countries use the same companies.