The gang at Weld Racing makes and sells a bunch of different wheels that both drag racers, and more recently road racers, have used for many years. They get asked the same question over and over when it comes to people looking to order wheels so they have whipped up some videos that explain some of the common things that a person needs to know for ordering and maintaining their wheels. This particular video is explaining how to find the stud diameter and thread pitch. They also talk about what the studs are for and what some of the parts are for.
The studs hold the wheel to the mounting surface so that the power from the engine gets to the tires. The stud and the thread pattern are important and determine exactly how much torque that you need to apply to get the lug nuts properly tightened and to keep the lug nuts tight enough to maintain friction. That allows the car to move forward with the wheels properly attached for safety. If the lug nuts get loose it could sheer the wheel studs off and send your wheel flying.
Determining the stud diameter and thread pitch can be done with a little measuring kit seen the in the video that you can simply screw your lug nuts onto and find the thread pattern and diameter simply. The same thing can be done for the studs by screwing the same measuring kit onto the wheel studs. The other way is more difficult and has a thin metal plate that measure diameter to tell you what diameter the stud is and then it can measure thread pitch by having the threads drop into little grooves on the side. This is straight forward and will help you stay safe. Check out the video for the full detail.