This means that you can basically choose if your creature comes back with a +1/+1 counter or a -1/-1 counter
# RB05_AD Devil's Knowledge 1BB Sorcery Until end of turn, set your hand aside
19961998 [ Pocket Monsters (19961998) ] (PDF), 2nd version, Waseda University, archived from the original (PDF) on 6 January 2018, 512 [Pokmon's main customer base is boys and girls aged 5 to 12.] : [ Game Freak: A Creative Group That Redefines the Global Standard for Gaming ]
A lot of sets have used it in different ways
The plan could not be more beginner friendly, you play creatures, attack, make more creatures, and repeat
(A player with ten or more poison counters loses the game.) Whenever another spider deals damage to a player, that player gets a poison counter