The act of asking "What's this?" before opening a link sent by a friend using a communication medium, such as AIM or text. This implies to your friend that sent the link that you either don't trust them, or you think their links are not interesting, usually telling them not to send you another link again, no matter how awesome it is. Also the friend wouldn't want to spoil the fun, so it gets them angry.
Julie: Check this out!!
John: What's this?
Julie: WTFUX U link filtering me for?