When do I get this error?

The "Missing 'use strict' statement" error is thrown when JSLint, JSHint and ESLint encounter a function that does not contain the strict mode directive, and none of whose ancestor scopes contain the strict mode directive. JSHint will only raise this warning if the strict option is set to true. Here's an example of a function that does not run in strict mode:

/*jshint strict: true */
function example() {
    return true;
}

Why do I get this error?

This error is raised to highlight a lack of convention. However, as JavaScript engines move forward, this error will increasingly be helpful as it should highlight areas of code that may not work as you expect them to, or may even cause fatal JavaScript errors.

A "use strict" statement is an example of a directive, which can appear as the first statement of a program or a function (ES5 §14.1):

A Directive Prologue is the longest sequence of ExpressionStatement productions occurring as the initial SourceElement productions of a Program or FunctionBody and where each ExpressionStatement in the sequence consists entirely of a StringLiteral token followed a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion. A Directive Prologue may be an empty sequence.

The "use strict" directive can be used to force the engine to conform to a strict subset of the language, as defined in ES5 Annex C. It has become something of a convention to run all JavaScript code in strict mode, to avoid falling into traps that are apparent in the non-strict language. See the previous link or the corresponding MDN article for the details of the differences in strict mode. You can fix this error by simply adding a "use strict" directive to the function, or to an ancestor function:

/*jshint strict: true */
function example() {
    "use strict";
    return true;
}

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. This message is treated as an error by JSHint which means you are unable to prevent it from being issued by ID.

In ESLint the rule that generates this warning is named strict. You can disable it by setting it to 0, or enable it by setting it to 1.