Responsive Website Design
Is the process of setting up your code to insure the same URL renders correctly for the device and viewport. 14,000 different view ports to code for. Using standard view ports for wide screen, desktop, tablet, horizonal mobile and then finally mobile. Keeping an eye on your Google Analytic to see which device is the most common for your website to render.
Get started With Mobile Friendly Website Design
SEO Starter Guide for Website Design