The use of mobile devices to surf the web is growing at an astronomical pace, but unfortunately much of the web isn’t optimized for those mobile devices. Mobile devices are often constrained by display size and require a different approach to how content is laid out on screen.

There is a multitude of different screen sizes across phones, “phablets”, tablets, desktops, game consoles, TVs, even wearables. Screen sizes will always be changing, so it’s important that your site can adapt to any screen size, today or in the future.

Google’s Responsive Web Design Basics