Journal Impact Factor (JIF) is trademarked by Thomson-Reuters. 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.Thomson-Reuters ranks, evaluates, and compares journals within subject categories and publishes the results in Journal Citation Reports. Rankings are calculated once a year and come out in June for the previous year's journals. Three years worth of data are required to calculate a Journal Impact Factor.
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. These are essentially the journals being cited the most often, according to Thomson Reuters. Unfortunately, some interdisciplinary areas, newer fields/journals, and many journals published outside the U.S. are not covered well by Web of Science. If you are looking for a journal that does not seem to have a Journal Impact Factor, the next best thing is to look for it in SCImago.