Some are large. Some are small. They are unsaved and need the gospel of Jesus Christ. Yet they probably have a church available in their own language and culture. They could go to church if they chose.
Most countries consist of several people groups.
Note that in a largely evangelized country, there may still be unreached groups. For example, Mexico has a significant church among Spanish-speaking people. But several of the smaller Indian groups still remain unreached. On the other hand, in the least evangelized countries, some groups may be reached. For example, while some people groups in Indonesia have churches, the large majority remain unreached.