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 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.