Hotter water extracts faster.
Light roasts need heat to taste sweet; dark roasts turn bitter when the water is too hot.
What it is
The temperature of the water when it reaches the coffee, usually between 88 and 96 °C.
Why it matters
Hotter water dissolves flavor faster. Light roasts are dense and need heat to taste sweet; dark roasts dissolve easily and turn bitter when the water is too hot.
In practice
No thermometer? Water that has just stopped boiling is around 96 °C; about 30 seconds later, with the lid open, it's close to 93 °C. Stay on the hot side for light roasts.