An explorer's reward is a view of tomorrow's possibilities.

Explore

You may play an additional land this turn.
Draw a card.

As he crested the final peak, Nidd gasped at the beauty laid out before him. Finally, he had found a place to rest his weary legs. A place to call home.

Foundations Jumpstart

Bloomburrow Commander

Outlaws of Thunder Junction Commander

Secret Lair Showdown

The Lost Caverns of Ixalan Commander

Explore's effect allows you to play an additional land during your main phase. Doing so follows the normal timing rules for playing lands. In particular, you don't get to play a land as Explore resolves; Explore fully resolves first and you draw a card, perhaps a land you'll play later.

If you somehow manage to cast Explore when it's not your turn, you'll draw a card when it resolves, but you won't be able to play a land that turn.

The effects of multiple Explores in the same turn are cumulative. They're also cumulative with other effects that let you play additional lands, such as the one from Urban Evolution.