WCAG stands for Web Content Accessibility Guidelines. It is a real standard with real version numbers and it reads like a tax code. Which is why most small business owners hear about it exactly once, in a demand letter.
Underneath the document there are four ideas. That is genuinely it.
Perceivable
Can somebody take the information in, through whatever sense they are using.
That is contrast good enough to read in a truck at noon. Alt text so a screen reader can describe your photos. Captions on video. Not relying on color alone to communicate something, because a fair number of men cannot distinguish your red error state from your green success state.
Operable
Can somebody actually use it, with whatever they have got.
Everything reachable by keyboard, in a sensible order, with a visible focus outline so you can see where you are. Nothing that requires a precise drag or a hover that a touchscreen cannot do. No carousel that moves on before somebody has finished reading it.
Understandable
Can somebody figure out what is going on.
Plain language. Form labels that stay visible instead of vanishing placeholder text. Error messages that say what to fix instead of just turning the box red. Navigation that is in the same place on every page.
Robust
Does it work with assistive technology properly.
This is the technical one. Real HTML elements. A button that is a button, a heading that is a heading, in order, without skipping levels. Screen readers rely on that structure and a div styled to look like a button tells them nothing.
The levels
A, AA, and AAA. AA is the one almost every law and settlement points at, and it is achievable on a normal build without heroics.
AAA is genuinely hard and mostly relevant to specific institutions. Do not let anybody sell you AAA on a five page site for a plumber.
The honest part
Nobody is perfect on this. Not the big companies with accessibility teams. There is always something.
But a site that got the four principles roughly right is in a completely different position, practically and legally, from a site where nobody thought about it once. One of those is a good faith effort. The other one is the one that gets the letter.