* Tapped cards can't block: A tapped card is essentially inactive. It can't perform its normal actions, including blocking attacking creatures.
* Attacking only affects creatures: When you attack with a creature, your opponent only has the option to block with creatures they control. A tapped card isn't a creature, and therefore can't be used to block.
* Creatures can still damage untapped cards: If your opponent has untapped cards, such as Planeswalkers, enchantments, or lands, your attacking creature can still damage them. However, tapping a creature only prevents it from blocking, not from being damaged.
In short, attacking with a creature against a tapped card is a wasted effort. The tapped card doesn't do anything to stop the attack, and your attacking creature can't attack it.
Example:
* You have a 2/2 creature and attack your opponent.
* Your opponent has a 3/3 creature tapped and a 4/4 creature untapped.
* Your opponent can choose to block your attack with their 4/4 creature, but they can't use their tapped 3/3 to do anything.
Let me know if you have any other questions about Magic: The Gathering.