Progressive scan was mainly found and specifically called out as a feature in games from the Dreamcast, Xbox, PlayStation 2 and GameCube era. It basically provided a cleaner image in a time before HD-consoles. In order to make use of Progressive Scan in games, you had to have a component cable connected to your TV and console.
Progressive scanning (or non-interlaced scanning) refers to the lines of an image being scanned in sequence as opposed to alternating between scanning the odd and even numbered lines which is how it is handled with interlaced, analog images.
On game boxes, it was also referred to as "Enhanced Definition" (EDTV) and basically means the game outputs at 480p with the p referring to "progressive (scan)". It does not, however, refer to whether or not the game outputs a 4:3 or 16:9 (widescreen) image. For example, The Legend of Zelda: Wind Waker on the GameCube supports progressive scan but is not widescreen enabled.
Many GameCube and PS2 games that supported progressive scan in their NTSC versions did in fact not do so in their respective PAL versions. With the Wii, the GameCube's successor, all games supported 480p across all regions while no longer referencing progressive scan but instead putting an EDTV logo on the back of the box.