Hey there. I'm tinkering around in Visual Studio 2017, C#, WPF. When I make a control and click on it, I see lines and handles. I tried googling some stuff, but I'm not getting the name right, so I can't figure out what they are. What the heck are these, and what do they do?

http://admiraladama.dreamhosters.com/uploads/visual_studio_handles.png