The KPI library is a storage of the KPIs that can be used for measuring the employee, organizational unit and company performance. Thus, there are three KPI types in the library:

Company KPIs
Org Unit KPIs
Employee KPIs
To access the KPI library go to Performance > Goals > KPI Library. Click the KPI name to view its details.
KPIs are created in the library and then assigned to employees and organizational units.