A refresh grant (sometimes called a top-up or annual equity award) is new stock awarded to someone already at the company. The original grant from your offer letter vests over a set number of years and then stops. Without refreshes, your equity pay would fall to zero at the end of that schedule. Refreshes stack on top, each with its own vesting calendar.
The effect is a layered income stream. Imagine a $200,000 new-hire grant vesting evenly over four years, plus a $60,000 refresh each year that also vests over four years. In year one you receive $50,000. In year four you receive $50,000 from the original grant plus pieces of three refreshes, roughly $95,000. In year five, the original grant is gone but the refreshes continue.
Refresh sizes usually depend on level and performance, and companies differ a lot: some grant generously every year, some grant only for promotions, some do not refresh at all. It is a fair question to ask during an offer, and the answer changes what year four and five actually pay.
The common mistake is projecting future income from the new-hire grant alone. Since each refresh vests on its own dates, it is worth listing them separately, so a plan knows which months carry a vest and how large each one is.