To replenish means to fill something back up after it has been used or emptied. When you drink most of your water bottle during soccer practice, you replenish it by refilling it at the fountain. When a store sells many bags of chips, workers replenish the shelves by putting out more bags from the back room.

The word suggests restoring something to its original, full state. A tired hiker replenishes her energy by eating trail mix and resting. A lake replenishes itself when spring rains refill it after a dry summer. Your body replenishes its oxygen supply with every breath you take.

Replenish implies maintaining a supply that gets used regularly. A library replenishes its collection by buying new books to replace worn-out ones. Parents replenish the pantry with groceries each week. The word carries a sense of renewal and restoration: you're bringing back what was there before and is needed again.