Several Apps with similar goals communicating with each other.
E-mail is an example for a (non-COVID-19-related) federation: many developers and operators have built and are operating e-mail systems independently of each other, but it is still possible to send e-mail from one system to another. This is because these e-mail systems form a federation.
In a COVID-19 context, multiple Apps from multiple App Creators could also form a Federation; for example, contact tracing information could be exchanged across the boundaries of any one App. This would support, for example, contact tracing across national boundaries with different Apps from different Public Health Systems.
See also Wikipedia entry on federation.