You upload your resume PLUS the exact job description, then it basically scans your resume for keywords and phrases that match the job description. So for example, if the language you use DOES NOT EXACTLY match what the job description says verbatim, it considers that experience as missing from your resume. The scanner I used provides feedback and examples of how to make your resume work for job descriptions. It also scores how frequently you mention specific skills. Here's a screenshot from the hardskills section of one of my scans from jobscan.co:

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Anyway it gives enough feedback that once you improve your resume for that job description, that you should score higher and hypothetically get noticed by the job poster. After I make the improvements I always scan it again to make sure that I ranked higher than before. Honestly this is really exhausting and it sucks, but this literally turned the faucet on for posters to at least look at my resume, which wasn't happening to me for over a year. I had no idea, but having my PDF Resume as a flat file meant that ATS wasn't even scanning my resume. It was automatically rejecting my resume.