The word yon has largely been out-dated and removed from modern speaking. Originally it was used as the third demonstrative pronoun after this and that. An example in using it is when it is said: this hat (meaning the hat closest to the speaker), that hat (meaning a hat close to the speaker but further away from 'this' hat) and finally yon hat (used to describe a hat further from 'that' hat yet still in view).
Originally there was even a 5th demonstrative pronoun, used for something out of sight. A colloquialism of yon is yonder which is still used in modern English. However I think that the English language is slightly impoverished without the word yon in our everyday speak and I have begun to use it, in the mild hope that it will one day be reintroduced into the English language.