It depends how your site is built. If it is a WP site, you want a responsive theme. These theme rearrange elements on the page so that they fit differently depending on the screen resolution. To see what I mean review this mobile responsive theme:
http://demo.studiopress.com/metro/
When you look at the site, make the window of your browser smaller or larger and see how the theme changes design.
So if your customers are viewing the site on a phone, they will we get a nice usable site.
Does that help?