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.