Understandable content means that the user can easily make sense of it. This clearly includes of reading and understanding the text of the page. You should strive to use proper grammar, correct spelling, and write to your audience’s expected reading level (or slightly below).
But this requirement also covers users understanding how to make use of interactive elements, especially graphical user interfaces. Making your content understandable benefits all users of your website, and is a staple to good web design.
Finally, it also means that navigation, especially finding information on your website - should be clear and understandable. Universities are rightly criticized for having difficult-to navigate web sites. No doubt you’ve encountered this yourself.
Consider that a staple how-to design manual for web development is titled “Don’t Make me Think”:
Note that this book is available for free to K-State students through the O’Riley for Higher Education library . It would be an excellent resource for you to draw from!