Viral Content is used to describe a piece of content that has become popular very quickly throughout the internet. A piece of content that is viral is being shared massively for better or for worse. It is hard to determine what content is going to become viral, so do not create content in hopes that it becomes viral. Create content in hopes that users will find it helpful and relevant!
Do you remember those funny YouTube videos with tons of views and shares? These are usually described as viral content. In other words, viral content is content that gets really popular really fast - usually by having viewers/ visitors sharing it to others soon after publishing.