It's quite simple to explain: if you use a "slow" card you lose your next turn.
some card ideas using "slow":
Lazy crowd: common, cost 7 recruits, slow, attack 14
Tower restauration: common, cost 4 bricks, slow, tower +10
Withdraw: common, cost 5/5/5, slow, discards your hand and receive eight new cards
Accumulation: uncommon, cost 0, slow, resources +6
Spite: uncommon, cost 5/5/5, slow, discard one opponent's card
Lazy apprentice: uncommon, cost 8 gems, slow, magic +1
Sleepy dragon: rare, cost 20 recruits, dragon, slow, durable, attack 40