A website is considered Mobile Friendly if it is designed so it presents itself well on a mobile device like a cell phone or a tablet. In addition, it should use navigation and present images and content that are easily displayed on a mobile device.
Mobile friendly websites use “responsive design” to ensure that the website is rendered properly for many screen sizes and resolutions. Responsive design tells the web server to display the site based on screen area (percentage of screen size for example) as opposed to strict pixel-widths. Another method is to serve an alternate “mobile site” to users who access the website via a mobile device. In this case, the web server checks the device type before serving the website to the user.
If a website relies heavily on scripts, video and audio elements, complex navigational elements, and large images, it may be difficult for users to navigate the site on a mobile device. Crunching images (reducing the size of images and graphics) is important for lowering page load time.
Google and Bing factor mobile-friendliness and mobile-accessibility in their ranking factors. As a result, it’s very important for websites to be movile friendly.