Ratio is coffee to water.
How many grams of water per gram of coffee. It sets how strong the cup is — not whether it's sour or bitter.
What it is
The ratio is how much water you use for each gram of coffee. 1:16 means 16 g of water per gram of coffee — 15 g of coffee for about 250 g of water.
Why it matters
Ratio sets how concentrated the cup is. Less water (1:14) gives a stronger, heavier cup; more water (1:17) a lighter one. It doesn't fix sourness or bitterness on its own — that's extraction.
In practice
Most pour-overs sit between 1:15 and 1:17. Weigh both coffee and water: a scale is the most useful tool you can add. Change the ratio only once the grind tastes right.