AES is an acronym for Advanced Entertainment System, the subtitle for SNK's Neo-Geo home system (as opposed to the arcade version, the MVS).
As the home and arcade systems are essentially the same (the only difference being the cartridges, which while containing both home and arcade code, are made in different sizes to prevent arcade operators buying the cheaper home versions), the use of "AES" has become common when describing games or systems for sale.
This distinction becomes vital in the second-hand market, as prices between versions of games can become quite extreme. While someone declaring they had an MVS cartridge of Metal Slug for sale might expect offers of around $40, an AES cartridge will sell for prices well in excess of $1,000. Naturally with prices being so expensive for the rarer AES titles, there are those willing to convert an MVS game into an AES one, although this involves the destruction of an existing game as a 'sac cart'.