The tracking ID is a string like UA-000000-2. It must be included in your tracking code to tell Analytics which account and property to send data to.
The first set of numbers (-000000, in the example above) refers to your account number, and the second set of numbers (-2) refers to the specific property number associated with the account.
If you're working with an App + Web property, you have a Measurement ID instead of a Tracking ID. A Measurement ID uses the format G-XXXXXXX, and identifies the data stream sending data to your App + Web property.