Access certification helps with a management of access rights. These rights also called privileges, role assignments, authorities or authorizations need to be assigned to the right users in the right systems at the right time. Access Certification means reviewing the settings such as assignments of roles to users to make sure that concrete employees have accesses to the systems they need.
Example
An employee works as a manager of a research department. He has access to various systems and databases. He gets promoted to senior management, where he is assigned to a new working group and gets access to different resources. During access certification campaign, the list of his privileges is reviewed and privileges which are not required any more are according to that removed.