Understanding Javascript Documentation

I know this is a silly question so please go easy. I'm having trouble understanding documentation in general such as this page: http://ift.tt/18585Yu

Can someone explain the terms 'callback' 'currentValue' 'index' 'array' (in that context) and 'thisArg' in very basic layman's? I'm sure it's something simple but it's just not clicking in my brain, and it's making learning the language on my own very difficult so I would greatly appreciate the help.