When another parker reports you in their space, the Parkable app updates your booking to that space and reallocates the other parker to your original space in the app.
For example: your original booking was in bay B54, the other parker had B71, the Parkable app has now swapped your bays so you both have somewhere to park, and don't need to move your cars!
The automated email is just a kind reminder to check you are parked in your correct allocated space before you park in the future.