Hi Michaela Pacesova - Tags are not automatically applied to Jive content. All tags are manually entered by end users in Jive.
Although I don't think this is exactly what you're asking, there is a feature where you can create a category that has tag suggestions. The feature is if you end up giving certain tags to a piece of content, Jive can recommend a pre-configured category that matches these tags.
Are you seeing something different in your instance with tags getting automatically populated? This could be achieved through some sort of customization.