What are cookies?
Cookies kept by iPhone or iPad apps and web browsers are used to store user access behavior and in most cases, to store login information. For example the Facebook app stores your user name and password in cookies which allows the app to remain logged-in to Facebook each time you run the app.
What are the consequences of deleting cookies?
If you decide to delete the cookies from your iPhone, iPad or iPod Touch, the next time you run the respective apps, you will need to log in again. Of course in this case, only affected are apps which store user account information.