CTR
CTR (click-through rate) is clicks divided by impressions, as a percentage. It measures how well content turns views into visits. CTR varies enormously by format and placement position, so compare like with like: story link taps against other stories, not against feed posts without links.
Worked example
500 taps on 25,000 story impressions is a 2% CTR.
Related: CPC · Impressions
Full context in the guides, or measure it yourself with the free tools.