The basic concept of a thread plug is to provide a quick inspection for machinists and quality inspectors to verify that the threads are within the accepted tolerance so that it interacts with the mating part correctly. This verification would be performed by threading the Go thread gage to the full depth of the feature without restriction, followed by the non-acceptance of the No-Go plug gage.
What may be a surprise to many is that there are cases when a threaded hole accepts the No-Go thread gage but not the Go gage. To fully understand how this is possible you must understand the difference between a Go and No-Go thread gage and how each of them interacts with the threaded feature.