Generally, the term "mobile compatible" means that a site can be viewed and used on a mobile device. This basic endorsement of a site for mobile use is different than another similar term, "mobile optimized," which means that designers have built in design features that make a site user friendly for mobile users.
The vast majority of sites on the internet are mobile compatible. However, many are not mobile optimized. Websites are often made predominantly for computer screens or other devices with larger screen sizes. The controls are not optimized for touch-screen interfaces or smaller viewing areas.
In order to promote mobile compatibility and mobile optimization, there is a feature called "responsive design" that is being built into internet systems. Responsive design involves creating websites in ways that support mobile use. Two main aspects of responsive design would be allowing for a better viewing experience on a smaller screen, and aligning or orienting controls where a mobile user can easily get to them.
Mobile compatibility and mobile optimization will be an enormous part of web design in the future. Currently, a sizable percentage of users access websites through their smartphones rather than through other kinds of devices. This leads to a lot of frustration with sites that aren’t easy to use from the mobile phone interface. Experts are predicting that as mobile devices continue to take over, companies and website managers will scramble to build in more mobile-oriented features and better mobile use design.