Recipes often call for cooling cookies on a rack. This is because when cookies are left to cool on the baking sheet, they continue to bake and may become overdone. A rack allows air to circulate under the cookies, cooling them quickly.
If you do not have a cooling rack, you may remove cookies from the baking sheet and allow them to cool on paper towels on the countertop. When using this method, you may notice that the paper towels absorb excess fat from the cookies, which isn't necessarily a bad thing! Also they cool slower than cookies on a rack.