The Serial Number is an 8-digit number (E.g. SN 0000 0001). You will find it on the back of your device itself.
The Tracker ID is a 15-digit number.
If you have already added your tracker to your phone, you can find its ID in the Tracker Settings (right under the tracker's name).
You can as well find your tracker's ID number in the box, right next to its PIN.