I'm not the plugin's author but here goes...
Remove redundant title attributes from page lists, category lists, and archive menus.
This reduces unwanted noise for screen reader and brailler reader users.
Helps to support skip links for all keyboard navigators.
Add skip links with user-defined targets. (Customizable targets and appearance.)
See above :-)
Add language and text direction attributes to your HTML attribute
Important info for screen reader users whose native language (and software settings) may be different to those on the site.
Remove the target attribute from links.
Opening links in new windows (especially without clear warning) can leave switch users effectively stranded with no way back to the site.
Force a search page error when a search is made with an empty text string. (If your theme has a search.php template.)
Again - important for screen reader and braille users. Otherwise all they effectively get is silence or misdirection.
Remove tabindex from elements that are focusable.
Tabindexing should be banned from all sites! It often seriously impacts on the ability of keyboard navigators to move around a site effectively and in the way that they want.
Strip title attributes from images inserted into content.
Again - reduces unwanted noise for screen reader users. Note: this will be implemented in core for WordPress 3.5 *happy dance*
Add post titles to standard "read more" links.
Screen reader users often hear links out of content, so they need a way to distinguish one "read more" link from another. Imagine trying to figure out which of 20 "read more" links was a link to the one Post that you wanted to read in full..
Add an outline to the keyboard focus state for focusable elements
Absolutely essential for sighted keyboard navigators so that they can see where they "are" in a page full of links.
Show the color contrast between two provided hexadecimal color values.
Visually impaired users need a text/background contrast of at least 4.5:1. Dyslexics, on the other hand, suffer badly if the contrast ratio is too high.
If you're interested, there's an Accessibility WP group at http://make.wordpress.org/accessibility/
And I recently rewrote the Codex's Accessibility page which includes a list of useful resources.