A character using a shield defensively can reduce damage they take in combat by d4 each round.
A Fighter can also choose to use their shield offensively instead of reducing damage. For one successful attack each round, roll a d4 and use the higher result of the shield or the primary weapon's damage dice.Example 1: A 1st level Cleric with 5 hp is hit by a Goblin spear for 6 points of damage. The character has a shield and rolls d4 getting a 2. The Cleric only takes 4 points of damage and is still alive with 1 hp.
Example 2: A 1st level Fighter attacks a Goblin and hits with his sword for 2 point of damage. The character has a shield and chooses to use it offensively this round and rolls d4 getting a 4. The Goblin takes 4 points of damage and is defeated.