Accessibility (a11y) means designing and building a site so that people with disabilities can use it too: e.g. screen reader users, people with limited mobility (keyboard navigation), or cognitive difficulties.
Good practices include: image alt text, colour contrast, logical heading structure (H1, H2), forms with labels and error messages, links and buttons that work with the keyboard. This often helps all users (e.g. on mobile) too.
In many countries, public services must meet accessibility standards (e.g. WCAG). Even for a simple company site it’s worth building in the basics from the start — it’s a matter of ethics and often better UX.