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.