The Impact Factor of Clarivate Analytics measures the influence of a journal by establishing the relation between the citations the journal receives in certain year, say, 2018, to the publications this journal had in the last two years (2016, and 2017.) This impact measures actually the average impact of an article in a journal, and in this way it neutralizes the effect of the type of journal measured (if it publishes mostly review papers that are usually cited more, or research papers only, or has many papers or few, or is published twice a year of is it a quarterly etc').
Impact factor used to measure the importance or rank of a journal by calculating the times it's articles are cited. The calculation is based on a two-year period and involves dividing the number of times articles were cited by the number of articles that are citable.
The impact factor (IF) is a measure of the frequency with which the average article in a journal has been cited in a particular year. It is used to measure the importance or rank of a journal by calculating the times it's articles are cited.
Each paper in a journal is cited several times, and the impact factor of a journal is calculated by dividing the number of citations in a certain year by the number of publication of this journal in the former two years. But it does not mean that the IF reflects the value of each paper in the journal, because there are papers that are cited more than others. So, the less cited papers can enjoy the score received by the highly cited papers.