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 (and you'll draw a card, perhaps a land you'll play later) first.

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 Oracle of Mul Daya.

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.