What is the OU active directory object?

Organisational Unit: A part of Active Directory used to Organise and Manage the objects of AD

An organizational unit (OU) is a subdivision within an Active Directory into which you can place users, groups, computers, and other organizational units. You can create organizational units to mirror your organization's functional or business structure. Each domain can implement its own organizational unit hierarchy. If your organization contains several domains, you can create organizational unit structures in each domain that are independent of the structures in the other domains.

The term "organizational unit" is often called as "OU" in casual conversation. "Container" is also often applied in its place, even in Microsoft's own documentation. All terms are considered correct and interchangeable.