Modernizr is an open-source JavaScript library that helps developers solve challenges faced in cross-browser web development by detecting CSS3 and HTML5 features supported by the browser of end users. It provides feature detection, which allows developers to test new features and provide fallback measures for browsers where the features are not supported. Modernizr introduces a more efficient approach called Feature Detection, which checks whether the browser supports the code and gives a "false" or "true" result based on that. The library can be used by adding a script tag in the HTML document or by installing it using npm. It adds classes to the HTML tag and prefixes unsupported features with a "no-" prefix, allowing developers to add alternative styles through CSS. Modernizr also provides polyfilling with conditional loading using YepNope, which loads scripts only when needed, reducing complexity and improving performance.