An Applicant Tracking System, usually shortened to ATS, is the system of record for hiring. It collects applications from every source you use, whether a careers page, a job board or a referral, and keeps each candidate's full history in one place. From there it moves people through the stages of your process, from applied to shortlisted to interviewing to offer to hired, so nobody gets lost in an inbox and everyone on the team can see where things stand.
Before the ATS, most teams ran hiring out of spreadsheets and email threads, which fell apart the moment more than one person was involved. The ATS exists to make hiring a shared, trackable process rather than a private one. It is the difference between knowing exactly who is at which stage of every open role and simply guessing.
A modern ATS does more than store applications. It parses each resume into structured data, ranks applicants against the requirements of the role, and keeps a clear record of every note and decision. The point of all of it is the same: to let a recruiter start the day from a shortlist worth reviewing instead of a stack of files to open one by one.