'Unbox' means to take something out of a box. For example, when you get a new toy and you open the box to play with it, you are unboxing it.
You pronounce 'unbox' like this: un-boks. The 'un' sounds like 'un' in 'fun' and 'box' sounds like 'box'.
The word 'unbox' is made up of 'un-' which means to take away or remove, and 'box' which is a container. So, it literally means to remove something from a box.