What does the "What's happening" view show?
The "What's happening" view shows you what other people have been doing recently that may be of interest to you, across all projects, based on your roles in each project. So, for example, if you're a team member in a project, you'll see what other people in your team have been doing, and if you're a product owner, you'll see what other people are doing to the backlog and what happens at a higher level in the project.
Note that you can also see what has happened in a specific project, sprint, team, backlog item, task, etc., by opening the popup dialog for that object and clicking "Show activities" at the bottom of the dialog. Almost all popup dialogs in Scrumwise have an activity history like this.