Whenever an opponent casts a spell, reveal the top card of your library. If it's a creature card, put it onto the battlefield. Otherwise, you may put that card on the bottom of your library.
If an opponent casts a spell, this ability triggers and is put on the stack on top of that spell. This ability will resolve (meaning that a revealed creature card will enter the battlefield) before the spell resolves.
You must put the revealed card onto the battlefield if it’s a creature card. If it’s not a creature card and you don’t put it on the bottom of your library, it stops being revealed and simply remains on top of your library.
A “creature card” is any card with the type creature, even if it has other types such as artifact, enchantment, or land. Older cards of type summon are also creature cards.