Non roster invites can be thought of us as the supplemental list of players you'll see in Spring Training. The guys on the 40 man roster (which you can find here on SBNation or here on Padres.com) are the guys that don't need a special invite. They have to show up. They are the country club members. These other guys are kind of like the official list of guests.
Some of these "guests" used to be in other people's clubs. They were recently signed as minor league free agents. They know they aren't guaranteed a big league job, but will fight it out for one and probably settling for a starting spot in AAA Tucson. And the others are the hot the prospects. The guys the team wants an extended look at and perhaps sees cracking the roster at some point this year, but probably not at the start of the season. Some of them are just about ready for prime time and will be in AAA with those minor league veterans and others will start a little lower, but show lots of potential.
The Padres announced the full list of non roster invites yesterday.
Padres Announce Non Roster Invites
Gaslamp Ball | Jan 8