"Go viral" is a fairly recent expression, and it means that something online (such as a picture or a video) has been shared, re-shared and re-posted. Therefore, many online users know about it and the picture/video/etc has become popular. Simply put, it has spread online like a virus.

"Has Gone Viral" . This Image Gains Popularity On The Internet..It's Spreading Really Quickly...And People Are Sharing It To Each Other...