What is access certification?

There are a few different types of access certification. You might be referring to the programs that allows organizations to manage risk by Identifying Modifying and Certifying User access to precious company resources, And comprehensive Access Compliance Management strategy. Which includes the following.

Identifying who has access to specific resources.
Displaying access rights in Business-Friendly terms allowing business managers to determine if User Access is consistent with policy.
Modifying where appropriate by automatically changing, disabling or deleting access rights without requiring an enterprise provisioning solution.

I hope this helps.