If an ability instructs a creature to explore, its controller reveals the top card of their library. If it's a land card, they'll put it into their hand. Otherwise, they'll put a +1/+1 counter on that creature, then choose to either leave that card on top of their library or put it into their graveyard.
Pathfinding Axejaw
If no card is revealed, most likely because that player's library is empty, the exploring creature receives a +1/+1 counter.
If a resolving spell or ability instructs a specific creature to explore but that creature has left the battlefield, the creature still explores. If you reveal a nonland card this way, you won't put a +1/+1 counter on anything, but you may put the revealed card into your graveyard. Effects that trigger "whenever a creature explores" trigger as appropriate.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Raoul Vitale
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
