A Security freeze is a tool that limits potential new lenders from accessing your credit report.  It is a tool to help protect against new accounts being opened in your name without your permission.

ChexSystems is just like a credit bureau report but for checking accounts.

Freezing your ChexSystems report largely prohibits ChexSystems from releasing information about you without your consent. Many banks won’t open an account if they can’t read up on your banking history to ensure you’re a good risk, so a freeze makes it more difficult for an identity thief to open a checking or savings account in your name.

In September 2017, Equifax, which is one of the three credit bureaus, experienced a data breach.  There is a very strong possibility that your personal identification data is available for sale on the Dark Web from that breach.