An error message has appeared :Enter the password for the Exchange account"
I have tried re-booting the Ipad 3, but the same message appears.
How do I get rid of this message and what is an Exchange account?
That would be Microsoft Exchange, which is a corporate email system. Have a look in your email settings and see if you have somehow started setting up an email account. If you delete any accounts not known to you, the problem should go away.