A tough cookie is a person who will do everything necessary to achieve what they want. http://www.usingenglish.com/reference/idioms/tough+cookie.html

A tough cookie is someone who can stand up for themselves, who is not a pushover. Usually a woman or a child, because those are the people you might expect not to have this quality. It's rather like the French 'rodé'. ('Cookie' is just American English for a British 'biscuit')

In English grammar it is called a Metaphor. A cookie that is baked for too long becomes very hard or tough. A person who is strong or doesn't give up very easily can be called a "tough cookie".

a tough person, but sometimes they use cookie as a means of being sarcastic, i.e. that person is not very tough or not very difficult to handle