https://www.uscis.gov/i-9-central/form-i-9-resources/handbook-for-employers-m-274
As discussed in the E-Verify website, it is a web-based system that allows enrolled employers to confirm the eligibility of their employees to work in the United States. The purpose of the E-Verify system is to provide employers with a technological solution for employment eligibility verification.
E-Verify employers verify the identity and employment eligibility of newly hired employees by electronically matching information provided by employees on the Form I-9, Employment Eligibility Verification, against records available to the Social Security Administration (SSA) and the Department of Homeland Security (DHS).
E-Verify is a voluntary program. However, employers with federal contracts or subcontracts that contain the Federal Acquisition Regulation (FAR) E-Verify clause are required to enroll in E-Verify as a condition of federal contracting. Employers may also be required to participate in E-Verify if their states have legislation mandating the use of E-Verify, such as a condition of business licensing. Finally, in some instances employers may be required to participate in E-Verify as a result of a legal ruling.
https://www.uscis.gov/i-9-central
https://www.justice.gov/crt/ssa-no-match-guidance-page