I’ve never read any of the Harry Potter books, but a reverse image search tells me this is fan art featuring two of the characters – Harry and Ginny. Presumably she’s showing him her latest magic trick. Using just an ordinary school tie and a completely conventional strap-on harness, she can make her magic wand disappear completely from view. It’s a neat trick, but I’m going to bet that Harry has a fair idea where she has hidden it.
I’m afraid I don’t know the original artist. I found it on the Felm Cyber tumblr (originally larger version available here).