The word 'only' means that something is unique or the single one of its kind. For example, if you say, 'I only have one cookie,' it means you have just that one cookie.
You pronounce 'only' like this: 'OHN-lee'. The first part sounds like 'own' and the second part sounds like 'lee'.
The word 'only' comes from the Old English word 'anlic', which means 'one-like' or 'single'. It has been used for a very long time to describe something that is the one and only.