A go/no-go thread gauge is an instrument used by manufacturers to check that threaded parts such as screws and nuts have been made to the proper specifications--that is, that they're of the right size and that they're threaded in the right direction.
Plug Gauges and Ring Gauges
Go/no-go thread gauges are used to test screws and nuts. The "thread" in the name of the gauge refers to the fact that both of these parts are threaded, or have circular ridges allowing the screws and nuts to screw together. However, since screws have their threading on the outside and nuts are threaded on the inside, two types of go/no-go thread gauges are needed. Plug gauges are threaded externally and are used to test nuts by screwing into them. Ring gauges are threaded internally, and screws are tested by being screwed into the ring gauge.
Go and No-go
Each go/no-go gauge has two ends: the "go" end and the "no-go" end. The go end is made to correctly fit the parts being tested. The no-go end is made to fit incorrectly: the ring is too small for a screw to fit into, for example, or the plug is too big to screw into a nut. Each screw or nut is tested in both ends of the gauge.
How It Works
While questions such as, "Is this screw as big as it's supposed to be?" and, "Is this nut threaded properly?" can be difficult to answer by just eyeballing a screw, the go/no-go thread gauge makes them simple and concrete. If the threaded part fits into the go end of the gauge and doesn't fit into the no-go end, it is well-made. If it doesn't fit the go end or does fit the no-go end, it is not of acceptable quality.