When you add a location to a task, you can set to be reminded when you enter or exit that location's region. If you tap on the map icon you'll be shown the region and you can then drag your finger to make the region larger or smaller.

To create a notification for a location in a task:
  1. Create a new task or edit an existing one
  2. Go to the location field and make sure that it says "Search or type a location" in the location field. If it does not, swipe to the left on the field to turn that on.
  3. Type in a location and let the search show you possibilities. Since location alarms require a geo-location, we need to use the found results, not just plain text. Alternatively, you may use the location button on the right to search and pick location spots.
  4. You can also use the location button on the right to specify region sizes.
  5. You can now choose between alerts for arriving, leaving, or none.
  6. Save the task.

How does it work?
Informant uses iOS' region notification features to provide notifications in a low-power method reliably. One major caveat to using location alerts is iOS only allows 20 at a time per app. This means that more than 20 will not alert. Also iOS will alert Informant - and therefore yourself - only after you've left or arrived within the region after a minimum of 20 seconds and within an indeterminate portion of the region to prevent spurious notifications.