Unique responses refers to the number of different responses to an item from candidates. For a question that requires human-marking, such as an essay question, it is likely that each candidate will create a unique response, since it is highly unlikely that two candidates will have written exactly the same answer to the question. There are likely to be less unique responses for a computer-marked item, such as a multiple choice question. A multiple choice question with two options, that allows the candidate to select one option or the other, will potentially present three unique responses. This includes 'not attempted' as a response, hence why it would be three unique potential responses. For groups, this will display a calculation from the combined responses of all items in the group.
When marking an item, if more than one candidate has given exactly the same answer as another (and therefore falls under the same Unique Response), all of the items with that response will be awarded the same number of marks.