Many people around me use the term "unreached," and often mean different things by it. It can get confusing. A fascinating collection of some of these meanings can be seen in The Oxford Dictionaries project: https://en.oxforddictionaries.com/definition/unreached.
“Unreached” has a technical definition that researchers and mission strategists understand and use. It is: “an unreached group lacks a church with the resources to evangelize the group to its borders without cross cultural assistance.”
So, some things reached/unreached is not:
A group is "unreached" if it doesn’t have an indigenous church that can do the job. There’s a natural next question: how do we know if the indigenous church can do the job? That question has been debated for a very long time, and no one has a consistent solution.
When someone says “we have to reach them with the Gospel,” they usually aren’t meaning what I've described here. They either have a logistical meaning (placing workers among a group) or an evangelistic meaning (we need to tell them the Gospel). When we respond, we need to keep this in mind. Encourage the passion, while understanding there’s a lot in that statement that would have to be unpacked in a strategic sense.
When I hear this on social media and other places, I try to just hear and encourage the passion, and leave the strategy and technical conversations for more appropriate venues.