This check determines whether the Farm Admin account is a member of the local administrator group. Giving the local administrator permissions to the Farm Admin account poses a security issue.
The Farm Admin account has all permissions in a SharePoint farm. Also, the Central Administration application pool runs in the context of the Farm Admin account. If the server is compromised, the attacker can read the username and password of the user running the application pool and use it to get data from SharePoint, install malicious software, and perform other actions that can damage the system and the environment. In order to minimize the impact area, the Farm Admin account should not have local administrator permissions on any SharePoint servers.
Please note that there is a condition where the Farm Admin account must have local administrator permissions, but only temporarily. This is during the provision of the User Profile Synchronization Service instance. Also, when you install a cumulative update or a service pack, User Profile Synchronization Service may be reprovisioned as a part of the patching process. In these cases, the Farm Admin account must also be a local administrator on the server running User Profile Synchronization Service instance, but only until the service is provisioned.
Verify that the Farm Admin account is not a member of the local Administrators group on any SharePoint server. To do so, open Computer Management > System Tools > Local Users and Groups > Groups and double click the group Administrators. If the Farm Admin account is a member of this group, remove it from the group.
Additional information can be found in the following articles: