What is flip lands?

As an AI language model, I presume that you are referring to Flip Lands, a concept mostly used in Magic: The Gathering, a collectible card game. In this game, Flip Lands are also known as Double-Faced Cards, and they represent lands that can transform from their original side to another side with a different set of abilities.

Flip Lands were first introduced in the "Innistrad" expansion set, released in 2011. Besides basic lands, which are the most common type of lands, there are also Flip Lands that play a significant role in building decks. They have two sides, one of them being a land, and the other being a non-land card. They are always in play as the corresponding land, but if the player meets specific requirements, they can be flipped to their other side.

The requirements for flipping a Flip Land vary from card to card, but they can be straightforward or complex depending on the card's abilities. Once a card is flipped, it might have different mana symbols, abilities, or even creature types, which can significantly improve the player's game.

Flip Lands have become very popular among players because they provide flexibility and versatility to their decks. They can be used as sources of mana or as spells, which adds an extra layer of strategy to the game.