To replenish something is to put back what has been used, so it is full or ready again. A shop replenishes its shelves. You replenish a fridge after a party, or a bottle after a long walk. It is a little more formal than “fill up” or “top up,” which is what most families say at home. People also use it for strength and energy: after rest or food, you replenish yourself. Though actually, the tidy idea “fill again” is a bit too tidy. You do not usually replenish a glass you just drank at the table — that is simply filling it. Replenish likes stores of things that run down over time: food, water, supplies, money in a fund. The feeling is restore, not just pour.