Click-through rate (CTR) is clicks divided by impressions. If your result is shown 1,000 times and clicked 50 times, your CTR is 5%.
Why it matters
CTR depends heavily on position (higher results get far more clicks) and on how appealing your title and description are. A low CTR at a good position usually means your result isn't compelling or doesn't match the search.
Example
Two pages both rank fourth. One has a vague title and gets 2% CTR; the other has a specific, helpful title and gets 6%: three times the visitors from the same ranking.
What to do
Find pages with many impressions but few clicks in Search Console, then improve their titles and meta descriptions so they match the search and promise a clear answer.