Thread/Tap tool
The Thread/Tap icon in the Part Design workbench can be used to define standard threaded features such as screw threads. Threaded sufaces can be external or internal. (Internal threads can also be defined with the Hole icon.)
The confusion about this icon is because nothing is shown graphically on the 3D model to represent the thread!
When you make a drawing, all the 3D threaded surfaces are projected as thread symbols, saving you alot of time.
Here's a Tip: if you're not sure what an icon does, click on it and hit the F1 button to go directly to the Help page on that particular icon.