煮饭 is a separable verb (离合词) meaning 'to cook rice' or 'to cook a meal' — 煮 is the action of boiling/cooking, and 饭 is the object (rice or food in general). When adding duration, measure words, or resultative complements, you split them: 煮了饭 (cooked rice), 煮过三次饭 (cooked rice three times), 煮好饭 (finished cooking the meal). The object can be omitted in casual speech: 你煮了吗? ('Did you cook?'). It specifically refers to cooking grains (usually rice) in water, not stir-frying or baking.