Last Friday all credit agencies were mandated by law to make freezing and unfreezing credit reports free. There's no excuse now; everybody should freeze their credit.
What is a Credit Freeze
To get any sort of credit (credit card, car loan, mortgage, etc.), the lender will inspect your credit. They accomplish this by doing a hard pull on your credit report. By freezing your credit report, it denies the lender the ability to do a hard pull. Consequently, you will have to temporarily unfreeze it every time you actually apply for credit.
When you freeze your credit report, you set a PIN. In order to unfreeze it, you have to provide this PIN. Also, the credit agencies conveniently provide an auto-freeze date: you can set it to automatically freeze again at a future date of your choosing.
Why Freeze It?
We live in a post Equifax breach world. There were many security breaches before that one, but that was certainly the most damaging. Criminals have all the information they need to acquire credit in your name. By freezing your credit you prevent this.
If You Don't Freeze It, a Criminal Might
The only information a person needs to freeze their credit is their name, address, and Social Security number. If a criminal freezes your credit, then it's a a huge hassle to prove that you're actually you and get control of your credit report again.
Identity Theft Protection Services Can't Prevent the Theft
The most famous example of this is Todd Davis, the CEO of LifeLock. LifeLock is an ID theft protection service that used to run ads with his real Social Security number. But his identity was stolen at least 13 times!
Those services can't actually prevent identity theft. They can only warn you of the theft after it has occurred and help you gain back control of your identity and possibly recover some of the damages.
On the other hand, credit freezes can actually prevent identity theft.