What does this option do?

The JSHint freeze option is used to disallow the extension of native object prototypes. This is often viewed as bad practice and was a relatively common source of bugs in older JavaScript environments. In the following example we attempt to add a method to reverse strings to the native String object:

/*jshint freeze: true */
String.prototype.reverse = function () {
    return this.split("").reverse().join("");
};

The option does not prevent extension of native prototypes via the Object.defineProperty method since it allows you to safely extend such objects with non-enumerable properties:

/*jshint freeze: true */
Object.defineProperty(String.prototype, "reverse", {
    value: function () {
        return this.split("").reverse().join("");
    }
});

Side note: the implementation of string reversal above is naive because it fails to take into account the way characters are encoded internally in JavaScript. See this Stack Overflow answer for a great explanation.

When should I use this option?

The use of the freeze JSHint option will cause an "Extending prototype of native object: '{a}'" error, where "{a}" is the native prototype in question, any time it encounters an assignment that matches the rules discussed above. It's generally considered bad practice to extend native prototypes because it makes it easier to introduce bugs related to property enumeration and shadowing. See the article discussing the "Extending prototype of native object: '{a}'" error message for more details.

Note that this is an enforcing option which means JSHint does not apply it by default. If you do not explicitly set this option to true JSHint will allow the extension of native prototypes anywhere in your code.

Recommendation

Set this option to true (disallows the extension of native prototypes).