If your Web site uses applets, plug-ins, or PDF files, you should provide equivalent text that is accessible to a 'screen reader.'
A screen reader is a device that allows people with specific disabilities (blindness, for example) to be able to hear Web pages spoken out to them rather than being displayed on the screen.
If alternative text is not available, a screen reader will not be able to accurately convey what is being displayed by the scripts.
Here are some tips on how to design accessible content with: