This page shows a real-time stream of all activities received by the ActivityPub.com Inbox. You can learn about inboxes here.
That means you can see here all the (public) objects on the fediverse that include "https://activitypub.com/" in their Audience Targeting properties.
Try posting an Activity. How? Use ActivityPub. Use an ActivityPub client to post a social web activity that cc's https://activitypub.com/inbox.
No activities have streamed in yet.