They are computer systems that kickstart the initial sifting process to ensure the best CVs are put in front of the human recruiter. ATS systems analyze CVs against an uploaded job description and person specification. It is estimated that applicant tracking systems (ATS) filter out 70% of CVs before they are read by a human. This means you must read the job description and person specification, understand what the requirements are, and then reflect these clearly in your CV.
Be careful not to overdo it when using keywords as this can sound repetitive, aim to use a keyword two or three times.