>>96320762
Imagine you're playing MtG and there's a Creature that, on a 0-cost effect, can summon 2 Creatures from your Library. Before 2017, you could use those Creatures' abilities however you wanted.
Now, we have an Extra Deck in YGO. Think of this like a 15-card Commander Zone, where you can summon any number of them by sacrificing the listed materials on their cards. Synchro Monsters can be summoned from the Extra Deck by sacrificing 2 or more monsters whose levels exactly equal their own (including a specific type of monster, but that's not important), and some of the strongest ones when Rescue Cat was a problem happened to be Level 6. To put it simply, you could bring out your bosses off of the summon of 1 monster if that monster was Rescue Cat.
I'm not totally privy to how MtG plays out, but I hope that it gets the point across just enough.