2 things are inferred with this keyword phrase.
Mobile-Optimized WebSite Builder: A system for building web sites that are optimized for display on not only traditional systems but are also optimized for touchscreen phones and tablets.
WebSite Builder for Mobile Devices: A website builder for mobile specifically means using a mobile device such as a touchscreen phone or tablet to build the website. Building the web site can be further defined as creating and/or updating the web site from a mobile device.
While a website builder may output a website which displays on desktops & mobile, it may or may not be optimized to build the website using a mobile device.