I have had a bunch of clients who want to know what the three horizontal line icon is on their website when they look at it from their iPhone. Their main concerns are: "What is it?" and "Will people know what it means?" "WHAT IS IT?" The answers are, it is a symbol that represents "menu". If your entire menu was on a tiny screen, it would look really bad and would be very confusing. Using this symbol on smaller, responsive devices makes usability easier. "WILL PEOPLE KNOW WHAT IT MEANS"? Many people who use their mobile devices every day know what it means, and people who are starting to pick up on this new mobile symbol. I wouldn't worry too much about it, but if you feel very passionate, ask if there is a way to put the word MENU next to or in the icon itself. This symbol, and similar icons, are showing up in a lot of other places, including Twitter, Facebook and Microsoft responsive websites. Check out this article written by Jordan Moore written back in 2012 about the "Navicon".