The word 'extant' is not a word that is often heard as it is used in very formal English. The meaning it conveys is that of something that is quite old but still in existence. The following sentence can explain the context where you can use the word: Extant remains of the old fort. In the abovementioned sentence the fort is quite old but some parts of it are still existing and the term 'extant remains' is being used for those parts.
The word can be used for some other things also like manuscripts and documents. But in all the uses the meaning remains the same. The following sentence is another use of the word: I can find for you three extant copies of the paper.
The word can be used for some other things also like manuscripts and documents. But in all the uses the meaning remains the same. The following sentence is another use of the word: I can find for you three extant copies of the paper.