It fetches the page, collects every unique image URL referenced by an <img> tag, and sends a lightweight request to each one to confirm it actually loads. Anything that returns an error status, times out, or fails outright is reported as broken.
How to fix what it finds
Most broken images trace back to one of three causes: the file was moved or renamed without updating the reference, a CDN or hosting migration changed the base URL, or the source was a hotlink to another site that has since removed the file. Re-point the src at a working copy — ideally self-hosted — and re-run the check to confirm.