A screen reader is software that reads the accessibility tree of a page aloud and lets somebody navigate it by heading, by landmark, by link and by form control. It is not a browser plugin and it is not something you can simulate by turning your monitor off. Half an hour with one, on your own site, tells you more than any report, and every platform already has one installed. This page is how to run that half hour and what the failures sound like.
How people actually navigate with one
Not by reading top to bottom. They jump: headings first to find the section, then landmarks, then links or form controls. That is why heading structure and landmarks matter so much and why a page with one h1 and no other headings is exhausting to use even when every image has alt text.
How to run a screen reader test in half an hour
Turn it on, put your hands on the keyboard and do not touch the mouse. List the headings and see whether the page makes sense from them alone. Tab through the main flow. Complete your own contact form. Open and close a modal. Listen for anything unnamed, anything that says button button, and any moment where focus disappears.
What the failures sound like
A control announced as link with no name. A form field announced as edit text with nothing else. A modal that opens with focus still behind it. A menu whose expanded state never changes. Each takes seconds to hear and none of them appear in an automated scan, which is why 11 of the worked example's audit hours are manual.
Questions people ask about what is a screen reader
Which screen reader should we test with?
Whichever is on the machine in front of you. Testing with one is the difference that matters; testing with three is refinement.
Do we need a blind tester?
For a real judgement, eventually yes, and it is the single best money in this field. For finding obvious failures, your own half hour is enough.
Is visual impairment website accessibility just screen readers?
No: magnification, contrast and reflow at 200% matter to far more people than screen readers do, and are easier to get right.