What Does User-Level Security Mean?
User-level security in the context of Microsoft's Access, is a fine-grained level of restrictions and permissions to the database user.
User-level security allows the database administrator to group users with similar needs into common pools called workgroups. Permissions can then be granted to the workgroup instead of to individual users, easing the administration of permissions. Two default groups are provided, the Admins group and the Users group.
Techopedia Explains User-Level Security
Microsoft Access uses a database engine called Jet. Prior to Access 2007, all data was stored in an .mdb file. The Jet engine controls all access to any objects contained in the .mdb file. It does not matter how a user accesses the data (either through a front-end application or the command-line interface) because the permissions set by Jet will always be the same.
User-level security offers a very fine-grained level of detail in assigning permissions. For example, it can be defined that the Admins workgroup can read, edit and delete data from the Customer_Master table. Those in the Managers workgroup can view and edit data in the same table, but not delete it. Members of the Employees group can only view the table data.