Cookies are items that you can give to your workers to boost their productivity (like coffee for developers) by 50% or 100% if you purchase the Cookie III skill. You have a 2.5% chance to get a cookie with a 30 seconds cooldown as soon as you buy the Cookie I skill but you can double that chance by buying Cookie IV which lowers the cooldown to 15 seconds.
The effect lasts 15 seconds with Cookie I but can be upgraded to 30 seconds with Cookie II.
Cookie V automatically distributes your cookies to your most productive slaves workers.