Most job seekers assume an ATS (Applicant Tracking System) is making a judgment about whether they're qualified. In reality, the software is mostly doing pattern matching: extracting text from your document and checking it against keywords from the job description.
This means a strong candidate can be rejected for entirely mechanical reasons — a CV built in a table or text boxes that the parser can't read correctly, a PDF saved from a design tool that strips out text layers, or a skills section that uses different terminology than the job posting.
What the software is genuinely checking: contact information in a parseable format, section headers it recognises (Experience, Education, Skills), keyword overlap with the job description, and sometimes formatting consistency like date ranges.
What it is not checking: tone, personality, or anything you'd call 'fit' in a human sense. That judgment comes later, from a recruiter, once your CV has cleared the first filter.
The practical fix is unglamorous: simple single-column formatting, standard section headers, and tailoring your skills language to mirror the specific job description for each application rather than sending one generic CV everywhere.