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.
Tag ID
If you're working with a Google Analytics 4 property, you have a tag ID instead of a Tracking ID. Your tag ID identifies the web data stream to which you want to send your Google Analytics data.