An Applicant Tracking System (ATS) is the software a company uses to receive, store, search, and filter job applications. When you click "Apply" on a job posting, your resume usually lands in an ATS like Greenhouse, Lever, Workday, or Ashby — not directly in a recruiter's inbox.
The critical thing to understand is that for any popular role, no human reads every application. Recruiters search the ATS using keyword and Boolean search filters, and only matching resumes surface. If your resume doesn't echo the exact language in the job description, you can be filtered out before a person ever sees your name — even if you're well qualified.
This is why high-volume applying through ATS portals (an inbound application) often produces silence. The strategic response is to pair ATS submissions with direct outbound outreach to the actual hiring manager, so you're both in the system and in front of a human.