Does a land entering the battlefield under your control trigger landfall if you played it during an opponent's turn with something like Explore?

Landfall is an ability that triggers whenever a land enters the battlefield under your control (CR 702.163a). The trigger condition checks only that a land entered under your control — it does not care whose turn it is, whether you played the land or put it onto the battlefield by another effect, or any other circumstance surrounding how it arrived.

Playing a land is a special action (CR 116.2a) that a player may normally take only on their own turn during their main phase when the stack is empty (CR 305.1). However, cards like Explore don't let you 'play a land' outside your turn — they put a land card onto the battlefield directly as part of their effect, bypassing the normal play restrictions. Either way, the land entering the battlefield is what matters to landfall.

Because the trigger watches the battlefield — not the stack or the turn structure — any time a land card moves from anywhere to the battlefield under your control, every landfall ability you control will trigger. This includes lands put there by spells, abilities, or any other game effect, on any player's turn (CR 603.2).

Concrete example: It's your opponent's turn and you cast Crop Rotation (which you can do since it's an instant), sacrificing a Forest to search for and put a land onto the battlefield. The new land enters the battlefield under your control, immediately triggering your Retreat to Kazandu's landfall ability, even though it's not your turn.

So yes — landfall is completely indifferent to turn order. The sole requirement is that a land entered the battlefield under your control (CR 702.163a, 603.2).