Just like money in Monopoly, Mana is required to do almost anything in Magic the Gathering.  On the upper right corner of a Magic card, you will find symbols indicating the amount of Mana required in order to cast the spell.  It also tells you what color of Mana you have to pay, White, Black, Blue, Green, Red or Colorless(represented by a number).

Tapping

When a permanent is used for the turn, it is turned sideways to indicate the fact.  This action is called Tapping.  When a card is tapped, you can’t use it until it has been Untapped (turned upright again).

You tap a card when you use a land to make mana.  Some cards have the ability to tap opponent’s permanents to prevent them from being used.

Every card is untapped at the beginning of your turn.

Producing Mana with Land

Almost all the land can produce Mana of a certain color.  When you want to produce Mana for your use, you tap a land and add one Mana of the specified color to Your Mana Pool.  Some cards would have the ability to produce Mana, too, which would be clearly stated in their Text Boxes.

Any Mana produced only last for one phase of a turn.

Distribution of Mana

Now you know how to produce Mana, you have to think about the amount of each color of Mana should be produced.  Every spell you cast requires you to pay Mana of a specific color or colorless or a combination of colors.  A Red card usually needs more Red Mana, but sometimes it requires other colors too.  When you build your own deck, you would probably focus on using Magic cards of one or two colors.  So that you could have to inherent advantages of the color of Mana.  Like Red Mana, it usually wins by acting quickly, destroying and burning along the way to gain quick victory.  Many of its creatures have the ability of Haste which allow them to attack first.  That way you must prepare more Red Mana for your own use.  Mana distribution and the combination of cards can be very tricky and do some really clever strategies.  You would learn more able it as you play and grow in the Multiverse, planeswalker.