What are Inactive Subscribers?
The most common explanation of an inactive subscriber is one who no longer reads your message, however has not yet taken steps to halt in getting your email communications.
For example, say you send a few emails a week to list of 100 subscribers and 50 of them only open your emails as well as click on your links. The other 50 never open your emails thusly never clicking on your links but never unsubscribing from your list. The latter half would be considered inactive.