In the 1980's there was a growing movement of people who wanted to view the original movie without the cropping. The first attempt to satisfy this group was through the introduction VHS and laser discs. For these mediums, the moviemakers simply reduced the entire image to fit into the 1.333:1 aspect ratio of a television. This method of displaying the original aspect ratio is called letterboxing and it had many drawbacks. The first drawback was that the resulting image was much smaller than a pan and scan version. The second was that it produced large black bars at the top and bottom of the image.
I lost the www link while copying and pasting this for you.
Just write the term in google...
In short. The reduction of the filmed image, without cropping, to fit completely inside the VHS proportion, but it leaves two spaces, on top and bottom of the image, (black bars).
I'll have to do some more research to find out how to properly say it Spanish. You see, here they tend to leave many film-making terms in English .