The open tag is hopelessly vague; based on the tagged questions I currently see, it can refer to:
Accessing files in any number of languages. In some languages, there are better terms for this (e.g, stdio). In others, it's a basic task that shouldn't need its own tag.
Opening windows, applications, and documents, or picking files to open, in many different unrelated environments. There's nothing in common to all these questions, as the answer will depend entirely on the environment.
Network ports being "open".
There are currently 825 questions tagged as open. Can we get some Trogdor up in here?