An Open Ended question is when you are given space to write your own thoughts about something, or to further clarify one of your responses. These responses are often used to better understand trends or to zoom into particular problems. For example, a large proportion of the people praised or complained about the exact same thing.
It is very important to answer these questions clearly and honestly. Again, your responses cannot be identified back to you, as far as the Client can see, you are number 411 out of 1000 respondents who are completely anonymous.