Board Thread:Love Live! School Idol Festival Lounge/@comment-32829166-20170820152416/@comment-11148831-20170820154453

Based on your given information, "plock" means "Perfect Lock". In EN, "Timing Window+". What these cards do basically is written as the card's Effect─ in your case: "For every 17 Rhythm Icons, there's a 29% chance that the timing window will ease for 6 secs.". This Effect will make all your Goods and Greats to Perfects for 6 seconds after activation.

You can obtain these cards via the most common way: Scouting. OR through exchanging Stickers for cards in Sticker Shop.