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 tracking ID is automatically included in the JavaScript snippet for websites, but also needs to be included in other tracking technologies like the SDKs and the Measurement Protocol for Analytics to work.

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.

* Nguồn: Google Analytics