Mobile friendliness can mean a multitude of things, depending on who you're talking to. It can be helpful to think of it in terms of three goals for improving your site's user experience: Presentation, Content, and Performance.
Make websites that work well on a variety of screen sizes.
Adjust your content for mobile users.
Think about what your users want to do at your site if they are on a phone. A great example of this is Alaska Air's website. Their desktop site focuses on getting visitors to book trips. Mobile users, however, are probably more interested in checking-in for a flight or seeing if their flight is delayed. They've adjusted their site's content to reflect this, and it meets the needs of mobile users.
Give your users a smooth experience, even on a slow connection.
Though things have been getting better in recent years, browsing the Internet over a wireless data connection can still be pretty painful. This makes it more essential than ever to practice good performance practices, only sending the user the bits they will actually need.
While not strictly a part of the definition of being mobile friendly, defining who your target audience is makes these goals much more concrete. For example, it is absolutely critical to keep in mind which browsers and devices you will target when picking a mobile strategy. If your audience is full of early-adopters, you can focus on tablets and smartphones with standards-friendly browsers. On the other hand, if many of your site's users are on devices with less capable browsers, that may eliminate certain strategies as viable options.
Originally published on 4 May, 2011 on the Mozilla Webdev blog as "Approaches to Mobile Web Development Part 1 - What is Mobile Friendliness?", by Jason Grlicky.