what do css classes like my-2 my-lg-0 mr-sm-2 do in bootstrap 4

Question

I've read through other queries and the docs for bootstrap, but I still don't understand what these classes do to the elements. We appreciate any assistance! I'm merely attempting to reduce the size of my search bar to 1/4 of its original size on a large viewport, 1/3 of its original size on a medium viewport, and 100% of its original size on a tiny viewport.