Accessibility standards are a universally agreed-upon set of guidelines which make a website usable by people with various disabilities. This includes those who are blind, colorblind, with limited physical mobility, with seizure disorders, and more.
Many of those with disabilities use assistive technologies to surf the internet, so bringing your website up to ADA compliance means in great part that your website is compatible with these technologies. Common assistive technologies include screen readers, refreshable Braille displays, text-to-speech outputs, etc. Your website will also need to be navigable via keyboard only, among other concerns.