I am an Angularjs rookie. Recently I am reading guide in Angularjs website and get some confusion.

In angularjs API Document about `ngRepeat`, it says:

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> If you are working with objects that have an identifier property, you should track by the identifier instead of the whole object. Should you reload your data later, ngRepeat will not have to rebuild the DOM elements for items it has already rendered, even if the JavaScript objects in the collection have been substituted for new ones. For large collections, this significantly improves rendering performance. If you don't have a unique identifier, track by $index can also provide a performance boost.
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I don't understand this content:

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> Should you reload your data later, ngRepeat will not have to rebuild the DOM elements for items it has already rendered, even if the JavaScript objects in the collection have been substituted for new ones. 
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What this sentence mean? Can you give me an example?
Thank you very much!