To replenish something is to refill it. If you have a pet pooch, you’re probably obligated to replenish his food bowl to avoid being barked at incessantly.
Use replenish when talking about something valuable or desirable, such as money or food. For example, you might discuss replenishing your bank account, but you wouldn’t speak of replenishing a trash can. You can also use replenish in a joking way to show that something is of value to you; by noting that the jar of jellybeans on your desk needs to be replenished, you indicate that you require jellybeans to survive your day at the office. We get that.