The Inbox is the place to store all your to do’s until you know when to do them. You can use it like a daily journal, you can just dump all your thoughts there and at a later point try to plan them into your day.
You can find the Inbox under the 📥 button at the top (on iPhone) or in the sidebar (on iPad).
Create an Inbox task
Inbox to do’s are similar to any other task, just without an assigned day. You can create a new Inbox task, by tapping the ➕ button in the top-right corner of the Inbox. This will set up the task, to already be in the Inbox. You can see this, by the In Inbox in the When? section.

After tapping Create Task the to do will automatically be added to your Inbox.
If you want to move an existing task to your Inbox, you can edit it and then tap the More… button, next to the When? and select Add to Inbox. After you save the changes, the to do will appear in your Inbox.
Add a task to your timeline
Once you know when to do a task, you can add it to your day plan, by tapping the ➕ next to the title of the task. This will open the Edit window and allow you to enter a time and day for the to do. After you hit Add Task, your task will be removed from the Inbox and appear in your schedule.

Import Reminder to do’s into the Inbox
If you're using Structured Pro, you can automatically import new to do’s from the Reminders app into your Inbox. From there, you can add them to your schedule, like normal tasks. To learn how to enable the Reminders integration, checkout How to Import your to do list into Structured.
Pro Tip: Template Tasks
Sometimes you have a task, which irregularly appears again and again. Instead of creating a new one every time, you can just create a template task in your Inbox and copy it into your day plan when you need it. To achieve that, simply create a normal task and leave it in the Inbox. Instead of using the ➕, you can tap the title and then select More. This will open another menu, where you have the option to copy a task. Here you'll have to select time and day and tap copy task.

That way, the template task will stay in the Inbox and you can do this process over and over again.