This trophy is titled "Voyeur" and it's for spectating 20 player matches online. In order to spectate a match, first the host must enable spectators. Secondly, there must be 2 people in the lobby for you to spectate. Lastly, you must do a custom search and use triangle to join the game in the "Spectator Lobby". You should see an eye next to your name in the lobby if you've done this correctly. Simply spectate 20 matches and it should unlock after your 20th match.