Active & Inactive leads are pre-built filters.

You can access them on the left hand side navigation:

Actively Searching  - A list of contacts who have looked at at least one property, or have been created as a new contact/lead within the past 90 days.

Not Searching  - A List of contacts who have not looked at any properties, or are older than 90 days.

While reviewing your lead lists, you can also quickly access the Searching/Not Searching categories at the upper right hand side: