A reinforcement will add a fifth hero to your party. meaning your party will be much stronger! If the reinforcement hero has special skills, you will be able to use them. If the reinforcement hero has a combo skill with one of the heroes in your party, you will be able to use their combo skill as well.
After completing the dungeon, you can send the reinforcement a friend request. When you have friends and you use their heroes as reinforcements, you will also benefit from their friend skill, which is the leader skill of their hero. Those leader skills may boost your stats depending on the skill, just like your own leader skills do.
When fighting a Titan you can join a guild and use guild members as reinforcements, which will not only activate the friend skill, but also boost your party's attack power by 25%. However, using a reinforcement in a Titan battle comes with a cost. The reinforcement will cost two more chaos energy to enter the battle.