The basic definition for Understandable in WCAG 2.0 terms relates to making navigation consistent on all pages of a website, making the language of the page or document clear, defining acronyms and abbreviations, and providing assistance with common errors that could be made on forms.
To take this a step further, though, you should consider making sure your content is well written and well organized, and that it follows good stylistic guidelines. This will go a long ways towards helping others to comprehend the information in your websites and materials.