Elemental Affinity

Starting at 6th level, when you cast a spell that deals damage of the type associated with your draconic ancestry, you can add your Charisma modifier to one damage roll of that spell. At the same time, you can spend 1 sorcery point to gain resistance to that damage type for 1 hour.

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What does this mean exactly?

Let's say your CHA is 18, so you get +4; let's also say you have gold dragon ancestry (fire related).

This allows you to deal +4 to ONE damage roll of a fire damage spell. So your fireball goes from 8d6 to 8d6+4!

The "one damage roll" bit can be confusing. 

For example, I'm a gold draconic sorcerer with a 20 in Charisma. I cast Scorching Ray. I create three rays that deal 2d6 damage. One of these rays can deal 2d6+5, the others deal 2d6. As I read it, you apply the modifier after the hit (since we don't typically roll damage for attacks that don't hit, etc). So it's a bit like a smite: If you're making an attack roll, you can apply it to an attack that hits.

A secondary example would be a blue draconic sorcerer with a 20 in Charisma casting Witch Bolt. On one of the damage rolls (either initial attack, or continued damage as you continue to use your action to keep the connection), you'll roll 1d12+5. The other rolls will still just be 1d12.

In short, Davothree describes possibly the best instance of using this ability: AOE spells which require exactly one damage roll, where your Charisma modifier is essentially counting as upcasting the spell one level and guaranteeing a roll of your Cha mod on the extra die.

Thanks!