Matte painting is the creation of imaginary or realistic sets in movies. It grants access to places that cameras cannot reach and helps build fictional universes. Mattes can cover a full frame or just a portion and are inserted in the post-production process.
Matte painting still exists today but in a digital form. Whether traditional or digital, both required talented artists to go above and beyond.
In the film age of cinema painters would have to make additional considerations such as the distortion of a lens or the way a particular film stock renders colour.
Once you start looking for matte's you are going to see them everywhere!