Web accessibility means building your website so that people with disabilities, in vision, hearing, movement, or attention, can perceive, navigate, and use it as fully as anyone else. In practice that is things like text a screen reader can read aloud, captions on video, enough color contrast to read in bright light, and a site you can operate with a keyboard alone.
For a small business, there are two honest reasons to care. The first is simple: a meaningful share of your potential customers have a disability, and a site they cannot use is a door held shut. The second is that accessibility has become a legal expectation in more places, often measured against a shared standard called WCAG, and it increasingly overlaps with ADA compliance for US businesses. This is background worth knowing, not legal advice; a lawyer is the right person for your specific situation.
The good news is that most accessible choices are also just good building: clear structure, readable type, labeled buttons, and a layout that already works on any screen the way a responsive design does. I build accessibility in from the start, because a site cannot serve every customer if some of them cannot read it, hear it, or click through it.