A page is read by people who cannot see it, cannot use a mouse, or cannot hear audio. The markup decides whether those readers can follow it, and most of the fixes are small and specific: names, order, and alternatives.
About 1 min of reading
Fix an icon button.
Icon only
Before you run it, say out loud what it will print. Then open the answer.
Named
Before you run it, say out loud what it will print. Then open the answer.
Everything reachable by pointer has to be reachable by keyboard, in a sensible order. Testing the page by tabbing through it takes a minute and finds most of what an audit would report.
Text against its background needs enough contrast to be read, and that ratio is computable. Choosing colours by eye is where the failures are, and checking the ratio is where the fix is.
An image that carries information needs those words in alt. An image that is decoration needs an empty alt so a reader is not told about something meaningless. Both answers are correct and both are choices.
A reader follows the order the elements appear, not the order they are painted. Reordering visually with CSS while leaving the source order wrong is the bug that makes a page confusing.
A button whose only label is an icon gives a reader the word 'button'. The accessible name is what is announced, and it belongs in the markup even when the design shows only an icon.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.