What an ATS resume checker actually does
Applicant tracking systems score resumes primarily on how well the text of your resume overlaps with the language of a specific job description. This checker simulates that step. It extracts the most weighted keywords and short phrases from the job description, scans your resume for them, and reports a single match score along with the keywords you cover and the ones you are missing.
This is not a substitute for a human recruiter reading your resume — but it is a reliable stand-in for the first automated filter that most job applications go through. If your resume fails this check for a specific role, it usually also fails the real ATS filter for that role.