A responsive theme is one that will adapt and be viewable in any device it is on. Whether your reader is on a large desktop computer screen, a tablet or a mobile phone the site will be viewable and useable.
This doesn’t mean that a user can just “make it work”, but that the site actually knows what type of device a user is on and sizes the website accordingly.
According to Cisco’s Global Data Traffic forcast “By the end of 2013, the number of mobile-connected devices will exceed the number of people on earth”. That seems like a good reason to be sure your website can be seen easily by mobile users?
Do you want a Responsive Theme? You can get a great one from the StudioPress Theme of the Month - Minimum Theme