The Journal Impact Factor can be found in the Journal Citation Reports (JCR) database; it is only available for journals indexed in Clarivate's Web of Science database. The JCR defines Journal Impact Factor as "all citations to the journal in the current JCR year to items published in the previous two years, divided by the total number of scholarly items (these comprise articles, reviews, and proceedings papers) published in the journal in the previous two years."
The Journal Impact Factor is commonly interpreted as a way to determine the "importance" of a journal, but the Journal Impact Factor is problematic, as is its application.
A few criticisms of the metric include the narrow scope of indexed journals in Web of Science, underrepresentation of international research because of a bias toward English language publications, and not recognizing the citation patterns of different fields of study. In addition, the citations on which the JIF relies lack context; citation doesn't necessarily reflect impact.
In short, the JCI is field normalized, meaning you can compare journals across disciplines. Need details? Here's Clarivate's explanation.