Click-through rate is the percentage of people who saw your listing in a search result and clicked it. Impressions in, clicks out. If a page is shown a hundred times and clicked five times, its click-through rate is five percent. Google's Search Console reports it for every page and every search, free.
The same term applies to adverts and emails, but in a search report it means organic listings. What counts as a good rate depends entirely on position. The top result takes a large share of clicks and page two takes almost none, so compare a page against where it ranks, not against a universal number.
