Activation rate is a way of measuring the success of a campaign, as well as the effectiveness of an email campaign by the number of users that clicked on a specific Short URL or an ad by the number of people that scanned a QR Code printed in it.
Activation rate indicates the percentage of people who upon seeing your campaign/QR Code actually engage with it by scanning the QR Code (or clicking your Short URL etc.)
Under the N/A you will notice the link, "Set Audience Count". Your total campaign audience is used to calculate the activation rate of the campaign across print, web, mobile and social media channels.
So for example, If took out an ad in a magazine with a circulation of 1M people and had 100,000 scans of your code your activation rate would be 10%.