The bin icon allows you to remove devices from your Device Tracker Plus account. If you no longer use the device, you can use the bin icon to remove if from your list and them you'll be able to to add a new device.
When you click the icon, you'll then see this prompt:
If you're 100% sure that you want to remove the device from your account click "OK". If you've clicked the bin icon by accident, just click cancel and the device will still be on your account with tracking enabled.