Like Speedstrength said above compressed is just a way of saying a formation is "tighter" together and inside the numbers just off the o-line. Usually the personnel will stay the same, unless the compressed formation is a certain package. Some common formations would be Doubles tight, trips tight or bunch, tre tight, etc. Why you do this is to typically get teams to check into some sort of coverage depending on how you line up formation wise and now you can scheme up better 1v1 matchups for your playmakers. Defense's usually check into the same coverage for certain formations they rarely see, unless they scheme up differently week to week. Empty check Cover 2, Quads check Cover 3, tight check Cover 2 or 3 depending on formation. This makes it easier on the defense, especially the Db's to get lined up. Alignment comes first, you can disguise and trick things up later. Sorry I kinda rambled and gave you the offense and defense perspective lol.