This is the icon directory. An icon is a picture with a purpose, i.e. it denotes something or is used as part of a navigation bar to indicate that clicking on it will cause some action.
Icons which are used by a number of diffferent pages are kept here for two different reasons.
1. If you need to change the icon you do not need to change every copy of the picture that you have.
2. Most browsers will use a cache which means that if they see the icon once they can use it again from their memory without having to ask the server for a new copy of the icon.