Journal Impact Factor (JIF) was originally trademarked by Thomson-Reuters, then sold to Clarivate. To have a JIF, a journal must be included in the Web of Science Core Collection.
JIF is a measure of the frequency with which the "average article" published in a given scholarly journal has been cited in a particular year or period and is often used to measure or describe the importance of a particular journal to its field.
Why does having a Journal Impact Factor matter? In some disciplines, journals indexed by Web of Science are considered to be higher quality journals...in part, because Web of Science only covers "core" journals in each discipline. The higher the ranking, the more often they are being cited, according to Clarivate. Unfortunately, some interdisciplinary areas, newer fields/journals, and many journals published outside the U.S. are not covered well by Web of Science.
How to find a Journal Impact Factor: There are several possible ways to locate them, but the single best way is to use Journal Citation Reports (see link below).
If you are looking for a journal that is not covered by Journal Citation Reports, it does not have a Journal Impact Factor. The next best thing is to look for a similar type of journal quality rating in SCImago or other tools linked on the Journal Rankings tab.