What's JSLint?
JSLint is the original JavaScript syntax checker.
This tool finds common errors in your JavaScript and enforces a coding style favored by its creator, Douglas Crockford, who literally wrote the book on JavaScript.
Advice: Use ESLint
While JSLint is the grandfather of all JavaScript syntax checkers, ESLint is what you should be using now. ESLint is far more configurable, checks for more issues, and has really easy-to-understand documentation. Most importantly, ESLint supports the newest JavaScript standards like ES6.
CodeKit 3.0 supports JSLint mostly for backwards-compatibility. As of 2016, ESLint is the industry standard in JavaScript syntax checkers. You should move to it immediately.
That said, this recommendation is in no way a condemnation of JSLint or Doug Crockford. It's just that the state-of-the-art has advanced since JSLint was created and modern JavaScript requires a more expansive syntax checker. Using JSLint today puts you at a disadvantage.
Using JSLint
Please see the help page for ESLint. Both tools are similar and the instructions for ESLint mostly apply to JSLint as well (though there are far fewer options for JSLint).