During your turn, you can put down one land card. To release the mana from that land, you turn the card sideways, also known as "tapping" the card. One tapped land card gives you one unit of that type of mana that you can spend to cast a spell.
Any card that is not a land card is a spell card. There are different types of spells; for example, there are creatures, instants, sorceries, and more. Learn more about spells.
A spell card shows its mana cost in the upper-right corner. A mana symbol in the corner represents one unit of that type of mana. A number in the corner means that in addition to the mana indicated by any symbols, you need to tap that number of land cards, but those land cards can be any type. An X next to a mana symbol means that the effect of the card changes depending on how many of that land you tap.