From what I know, it's where you're part of this "company" or "entertainment" to sing in a group that the company provides you. So basically if you're interested in singing or dancing and want to show case your talents on AFF or Youtube, you would audition for the entertainment. Then, they'll see if you're fit or if you're good enough for the group and they'll put you with two or more people. They will give you a song and I guess your lines. You'll record yourself singing those lines and send them back, where the "company" will give to mixers, who're in charge of putting all the different recordings into one. Once the song is mixed, they'll send it to someone who knows how to edit videos and they'll finalize everything. So basically, it's like AFF's version of K-pop groups.
Considering that I'm not too familiar with entertainments on AFF, the things I'm saying can be wrong. To put it more simply, you can think of it as getting together with a group of friends/strangers to make a cover of a song and posting them online.
