ADA compliance, for a website, means building your site so people with disabilities can use it: someone who navigates by keyboard, relies on a screen reader, or needs larger text or stronger color contrast can still read your pages and do what they came to do. The Americans with Disabilities Act is the US law the phrase points to, and while its application to websites is still being worked out in the courts, the practical standard most people follow is the web accessibility guidelines known as WCAG.
For a small-business owner, the reason this term keeps coming up is partly the right one, that a site everyone can use is simply better, and partly a wave of demand letters that has made some owners nervous. That worry is understandable, and it is worth saying plainly: this is general information, not legal advice, and a lawyer is the right person to speak to your specific situation.
What owners typically do is build accessibility in rather than bolt it on. Be wary of the one-line “accessibility overlay” widgets sold as an instant fix; they often fall short of what they promise and can introduce problems of their own.
I build accessibility into the site itself: real text, proper headings and labels, keyboard navigation, and readable contrast. It is the same work that makes a page easy for a screen reader and easy for everyone else, and it is the honest way to meet the standard rather than paper over it.