Matte painting is the process of painting a piece of glass that is placed in front of the camera to allow you to replace or add in additional scenery or features. The advantage historically of course is that you don’t need to do any processing afterwards – what is captured on film at the time is the complete scene – no post production is required.
Now an art form relegated to the computer screen like so many others. There’s little advantage to be had in doing it this way anymore, apart from the instant feedback, which can be achieved in other ways.
Here’s an interesting website if you want to find out more.