ATS is an acronym for applicants' tracking system. It is an electronic recruiter that acts as a middleman between applicants and their employers or hiring managers. ATS helps to sieve through thousands of applicant resumes for the most qualified candidates based on the commands in the system.
Contrary to the belief of many people, ATS is not a machine. It is a type of software recruiters use to streamline the hiring process. The system scan resumes for keywords, elements, and information depending on the job posting.
For instance, if a job posting specifies the need for a copywriter with two years of experience, an ATS will search for the word 'copywriter' and pick out all the resumes of candidates that indicate this.
It may also search for 'two years', so even if you're qualified and your resume fails to portray you in such light, your application will be screened out. It is the sole reason people fail to progress into the interview stage despite having the required skills and experience for the applied job.