My 'npm run start' error mentions 'Module not found'. What does this mean and how do I fix it?

A 'Module not found' error indicates that a required dependency is missing. First, ensure the module is listed in your `package.json`'s `dependencies` or `devDependencies`. If it is, run `npm install` or `npm update` to reinstall or update your dependencies. If it's a locally installed package, double-check the path in your `require()` or `import` statement.