Hydration
Hydration is the weight of water relative to the weight of flour, expressed as a percentage. A dough of 1000 g flour and 700 g water is at 70% hydration. It is the base figure of baker's percentage, where every ingredient is measured against the flour weight, which is what makes a recipe scale cleanly to any batch size.
Higher hydration gives a more open crumb and a slacker, stickier dough. Lower hydration gives a tighter crumb but is easier to shape. For a first sourdough loaf, 65–70% is a forgiving range.
The same percentage behaves differently across flours: strong bread flour absorbs more than weak, wholemeal more than white, rye more than wheat. Hold back about 5% of the water and add it only if the dough will take it.