This is correct (should also be explained clicking on the "?" next to it).
If you forward emails to that address from the same email account you registered your sync account with, they will be parsed and if possible added to your synchronized deliveries.
The title of your email will be used as name for the delivery.
The text must be something like this (please note that only numeric tracking IDs will be detected, rendering it useless for e.g. UPS):
Tracking ID 0123456789
In many shipment confirmations, there are direct links to the tracking results of the used courier. These should be properly recognized and you won't need to have the text formatted like mentioned above. Also the numeric restriction doesn't apply here - it works pretty much the same as opening links with the app.