Here's how it works:
* Direct Attack: If your opponent has no monsters on the field, they can directly attack your Life Points. You cannot defend against this.
* Monster Battle: When a monster is attacked, you have several options:
* Battle: Your attacked monster fights back. The monster with the higher Attack power wins the battle.
* Tribute: You can tribute another monster to increase the Attack Power of your attacked monster (some cards allow this).
* Card Effect: You can use a card effect to modify the attack, defense, or even prevent the attack entirely.
* Let it be destroyed: If you do nothing, your monster will be destroyed by the attacking monster.
You cannot use another monster to block or protect an attacked monster.
Let me know if you have any other Yu-Gi-Oh questions!