Sell high, buy low.
Vesting allows you to invest some of your cookies in a real-life currency.
Your invested cookies will increase or decrease based on your currency's exchange rate*. You can convert them to another currency as often as you like. (*Exchange rates based on euro (EUR); cookies invested in euros will never change.)
What does it do?
Your virtual money is converted back to cookies and added to your total banked cookies, if you're in human form. This allows you to level up your human form at double the rate of all other forms — or even faster, if you invest wisely.