stands for Mac App Store . A .dmg with “MAS” in the name typically means it was originally downloaded from Apple’s official Mac App Store. Legitimate MAS downloads are tied to your Apple ID. If you found this file on a torrent site, forum, or file-sharing link, it is almost certainly a cracked, pirated copy .
In short, is the disk image for the legitimate, App Store-signed version of Final Cut Pro, updated to version 10.5.4. Final-Cut-Pro-10.5.4-MAS.dmg