A journal's impact factor (JIF) is a metric that measures how often articles published in a journal are cited, and is used to rank the journal's importance relative to others in its field.
Impact Factors are used to measure the importance of a journal by calculating the number of times selected articles are cited within the last few years. The higher the impact factor, the more highly ranked the journal. It is one tool you can use to compare journals in a subject category.
The number of citations to articles published in the last two years divided by the number of articles published in those two years.