This is your personal outbox. It's basically a list of the activities you've published. It roughly corresponds to the list of toots you see on people's profile page on Mastodon.

{
    "@context": [
        "https://www.w3.org/ns/activitystreams",
        "https://forgefed.org/ns",
        "https://angeley.es/as2-ext"
    ],
    "actor": "https://dev.openheart.work",
    "object": "https://",
    "type": "Reject"
}