The End Does Not Justify The Mean, What Does It Mean?

What does the quote "The end does not justify the mean" exactly mean?

The first time I heard the statement above, I wasn't exactly sure what it really means. Honestly. Our instructor wrote those words in the blackboard during our class and asked us, his students, the meaning. I was so glad he did not ask me directly, because if he did, I swear I would stammer and remain silent.

Our instructor gave us an example. I can't remember the exact words, but the idea was like this: John has a very sick daughter, but due to lack of money, the hospital won't admit his daughter. To get the necessary money, his only choice is to rob a bank. In this case, does the end (to save his daughter) justify the means (to rob a bank)?

If you were in my position that time, how are you going to answer? Do you agree that it is not always right to do something wrong to get to something right?