Imagine that you created a multi-page survey. You placed a blue arrow button at the bottom corner of the page and wrote instructions at the beginning which told users to select it in order to navigate to the following page. Someone experiencing visual impairment may not be able to find that arrow button referenced in the instructions based on the information you've just provided them.
If you're using visual or audio features as the sole means to navigate or make sense of the content on your site, someone relying on assistive technologies will have difficulty using your page. These characteristics might include color, shape, sound, or spatial references.
So How Do I Fix It?
The easiest way around this problem is to follow that little piece of advice your mother always gave as a kid: USE YOUR WORDS. Adding textual references in addition to visual and audio components will help make sure that everyone is able to use your site.
Let's go back to that survey example from before. You could simply label the button with the word, "Next," and then change the instructions to read, "Select the blue arrow icon labeled, 'Next,' to continue to the following page," and watch the survey results roll in. Please note that you don't need to remove the color and shape as visual indicators. These can be useful for other users, including some who may be experiencing cognitive limitations.
It's also worth mentioning that sometimes spatial references work in context. For example, your text may reference something "above" or "below" to indicate content further up or down the page. If the reading order for your site matches your spatial directions, and the language you're using is clear, those references may work. Just use your best judgment!