What is mobile? It's where you want to be. It's the world in your pocket, and it is your customer's always-on internet connection today, especially when they're on the move and on holiday. And it will increasingly be where your wallet lives, as use of mobile payments accelerates.
Mobile means handy devices that move around with people - smartphones, cheaper "feature" phones, tablets such as iPads or phablets, and even small-screened netbooks and gaming devices. These all have smaller screen sizes than the laptops and desktops people used to design for, and they may have less processing power and may be using a much slower and possibly more expensive internet connection. And they may also be used in poor lighting conditions and on the go, meaning properly designed mobile websites are needed to deliver usability and a good user experience.
What if you do not have a mobile friendly site? People may never see your site if its file sizes are too big to load, or they load too slowly. But then if your site does load but has not adapted to the smaller screen, people have to scroll endlessly sideways as well as up and down and the menus may be baffling or unusable and forms may not work. So effectively you are telling those people to go away and maybe come back another day on a pc if they feel like it and if they even remember to. Why should they when there are plenty of other mobile-friendly sites for them to use right now?
A mobile-friendly experience suitable to your site and objectives is essential and is a basic part of the design process now. For example, a separate, dedicated mobile site may be the right choice, and would make decisions about what content or features to include depending on context. In other words, a mobile train timetable site really puts the search function front and centre and gives way less importance to the stuff people are unlikely to use on mobile, and may have few images or just small ones. Alternatively a mobile site can be simply a built-in, mobile-friendly stylistic variation of your desktop site with the same content, as often happens with responsive design. In that case there is just one overall site to create, administer and maintain, but it does need proper thought and testing to create it right.
As a rough figure we are now advised to expect maybe a third of traffic to come from desktops and laptops, a third from tablets, and a third from phones. In some areas the percentage of smartphone traffic and search will be much higher, so if you aren't there you are just lost and you are telling people to get lost. Can you afford to ignore this?