Of Them' refers to a group of people or things that have already been mentioned. It is a pronoun used to refer back to the group in order to avoid repeating the same words over and over. It is often used in conversation or in writing to avoid being too wordy. For example%2C if someone were talking about their friends and wanted to refer back to them without repeating their names%2C they could say 'Of them%2C John was the tallest'.