The Inbox, which appears at the top of your Goal Map, acts as a catch-all for odd steps which don't necessarily associate with larger goals, or are waiting to be organised into their respective goal.
For example, if you add a step from the prompt at the top of the To-Do List page, it will go straight to the Inbox, because no context about which goal it belongs to has been provided yet.
You can either choose to assign to a goal, by dragging the step out of the inbox, or just leave it in the inbox and treat it like a regular step.
Unlike every other goal or step on the Goal Map, the Inbox can't be changed or deleted, so offers no shortcut menu or Edit button.