Garbage collection: Unclear what is considered an 'active' Revision

[harrymilne]

Describe the change you'd like to see

In the Revision docs it is noted that revisions are retained if 'The Revision is active and is being reference by a Route' but the term 'active' is not defined anywhere.

When is a Revision considered 'active'?

Additional context

As user I'm trying to understand how Knative garbage collection works and the fundamental concept of 'active' Revisions is not explained.