This is where basal (slow acting insulin) is adjusted to cover insulin requirements without food,
and bolus (fast acting / meal time
insulin) is dosed based on amounts of carbs
and proteins that will be eaten at any meal.
The ratio of how much insulin to how much carb / protein is determined over time and with experimentation.
My Dr first told me to use 1 unit of fast acting insulin for 25 g of carbs, and adjust from there.