Romans 12
19 Beloved, do not avenge yourselves, but rather give place to wrath; for it is written, “Vengeance is Mine, I will repay,” says the Lord. 20 Therefore

“If your enemy is hungry, feed him;
If he is thirsty, give him a drink;
For in so doing you will heap coals of fire on his head.”

21 Do not be overcome by evil, but overcome evil with good.

From reading a quick commentary, it appears that not everyone can agree with what “heap coals of fire on his head” means.

But here is what I believe:
From the Bible Knowledge Commentary:

Sometimes a person’s fire went out and he needed to borrow some live coals to restart his fire. Giving a person coals in a pan to carry home “on his head” was a neighborly, kind act; it made friends, not enemies.

The true generous thing is to keep one burning coal for yourself and give the rest to the person in need.

By giving our enemy this many burning coals they have to carry them the way burdens are carried in the Middle East: in a container on the head. Then they can go back and immediately bake their bread without having to wait for the wood to become suitable coals for cooking.