A tracker allows you to track the carbon emissions of a certain activity or group.
For example, if you would like to track the emissions of 2nd unit, you can create a tracker called "2nd Unit".
Each emission that relates to 2nd unit you can then assign the tracker to.
When you go to the Emissions Report, or if you filter by tracker on the Carbon Calculator table, you will then be able to see the total emissions for that specific tracker, in this case, 2nd unit.
You can find the Tracking field under your Production Details, where you can view all your trackers and their corresponding amount of entries.