Impression is a measure of the number of times a surveys has been displayed, whether it has been completed or not. Each time an survey displays it is counted as one impression.
Survey link placed on the web page can be clicked 744 times, but only 445 people can complete the survey. In this case number of impressions will be equal 744 and number of completed surveys 445.
NOTE. Term "impression" has been borrowed from marketing, where it refers to display of online ads.