Cookies Days allow you to set a time limit (in days) on how long the customer has to purchase after clicking on the affiliate link in order for that affiliate to still get commission.
For example, imagine a user clicks on one of your affiliate's links to go to your site on January 1st, leaves your site, and then finally decides to come back on February 15th to a purchase. If your Cookie Day setting is set for 30 days, the respective affiliate will NOT get the commission, since it took the customer 45 days to purchase.
Note: Regardless of Cookie Days, in order for sales to be tracked on your site, the user must purchase on the same computer and browser that they used to click on the affiliate link. Also, they must not have cleared their cookies.
Here's how you can change the length of the Cookie Days period.
Head to Manage > Offers within your account:
Next, select the offer which you want to increase or decrease the cookie day length for:
Scroll down and click Advanced Options. Finally, change the amount of Cookie Days here and hit "save changes":