In web science, bounce rate refers to visits on a web page where the visitor immediately leaves again. They have simply made a mistake, can't see what they were actually looking for, or the page takes too long to load. Modern web users are not patient people! You can check up on your bounce rates with tools like Google Analytics or StatCounter.
As to how much is normal, opinions vary, but anything lower than 40% is considered good. if your bounce rate is over 60%, you're doing something wrong - either you're using the wrong keywords, your UX is not good, or the content you offer is not visible enough (or not valuable enough). So if you want people to find your pop-up shop, the first step is to get them to come to the channel your shop is at!
Read more about optimizing your online pop-up shop here.