"A software application that enables the electronic handling of recruitment and hiring needs. An ATS can be implemented or accessed online at enterprise or small business levels, depending on the needs of the organization."
Most Fortune 500 companies use an ATS to filter resumes before a human ever sees them. These systems scan for keywords, formatting, and relevance to the job description. If your resume isn't 'ATS-friendly' (simple formatting, correct keywords), it may be automatically rejected.