The trackers allow you to aggregate some operations together in a group, so you can follow them more closely. This may come handy to follow expenses for which you expect a refund, or simply knowing how much you spent during your last holidays in the Bahamas.
For example, imagine you have been sent to Italy by your company. Once there, you paid your hotel room: in Skrooge, you can create an operation in category "Business > Travels > Hotel", and assign a tracker called "Business Travel in Italy".You can assign this tracker to all professional expenses you make there. Then, if all goes well, your company pays you back for all these these, and you can create an income operation, and also affect it to the "Business Travel in Italy" tracker.
By affecting trackers to operations that shall be reimbursed, you are able to follow completion of the reimbursement.
This, of course, works also the other way round: if someone lends you some money, you are able to monitor how much you still have to reimburse.
Double clicking on a tracker in the track view will open a new tab containing all operations attached to this tracker.
When you no longer need the tracker, for example because it has been fully reimbursed, you may simply delete it. Doing, this, however, will loose all information you have entered, and that may be of interest for future reference.
A cleaner solution is to close the tracker: when a tracker is closed, it can be easily hidden from the trackers view, using the dedicated option.