The basic definition for Robust in WCAG 2.0 terms relates to making sure that the code behind your web pages and electronic materials is well formed, so that it will work with the widest range of current and future technologies.
To take this a step further, though, you should consider striving for the principle of universal design, meaning that anyone can easily access your content from anywhere with any common, standard device. This is achieved by using standard technologies and avoiding specialty technologies (e.g. Flash, Java, etc.) that may not work well with all special assistive technologies like screen readers or keyboard-only input. A key aspect of universal design for websites and electronic materials is testing carefully to make sure they are usable in all common browsers and/or document readers.