Threads..... No need to be embarrassed if you don't know What in the World This Is. They are
screw threads, used to attach lenses together. (Lenses are screwed together just like a lid is screwed on a jar.)
If you have them on a camera (or a lens), this is what they might look like:
P.S. Now I know why I have the WITWIT badge!
(a. k. a. What in the World Is This Badge)
