Each credit reporting agency offers this option, at no charge …
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.
Note that, before this incident, it wasn’t consistently free. I remember it being free to lock, but costing $20 or so to unlock. A law passed in 2018 required credit bureaus to offer freezes and unfreezes (and to fulfill them within certain time frames) for free.
Also note that you might need to look for a “freeze” instead of a lock. Experian charges $25/month for their “CreditLock” service, for example, but they offer a free security freeze.