Need some explanations about "track by" in AngularJS

Before asking my question i have done some search with google and yep, i found 3.000.000.000 results, but if i ask you this question today it's just because after many hours passed to read tutorials and explanations, i don't really undertsand how "track by" works and what he does, as far as i've seen 99% documentation need you to be already comfortable with angular...

EDIT: My main goal is too use it with ng-repeat to add some precisions :)

And i'am not a pro, i'am new to angular, so if someone a bit more pedagogue and patient can answer me it will be great :/

(I don't say that those tutorials are pure crap, or something like that, i really find them hard too understand, for a newbie like me of course, :/)

Thanks you all :)