Tracking a task means you will receive notifications – depending on your settings – on different activities over the task. Typically when someone commented a task, finishes or changes the deadline for the task.
You can track the task if someone will assign it to you, or you can assign it to someone else, or you will create the task yourself.
How to stop tracking a task?
You can always unsubscribe from the watch list. There are several options: Through the context menu, on the task detail, the notifications window, or the Dashboard on the tab of all the tasks being watched.