With many of us using mobile devices such as phones and tablets to surf the internet, it makes sense that websites should be programmed in such a way that they are compatible on these platforms and all aspects work as they should do. Google announced earlier on this year that they are going to favour mobile friendly sites in terms of rankings above those that are not, in order to offer their customers a better browsing experience.
Making a website mobile friendly, is done easier if done from the start, but for those that already have a website that is not compatible on such devices it can often easily be updated. There are many ways in which to make a site mobile friendly and often the easiest and cheapest way id to have a mobile version of the site which is only shown to mobile users. This allows you to create simply navigations and show reduced content if necessary when space may be an issue.