Some unscrupulous publishers may advertise an impact factor that has been falsified or garnered from a non-reputable source. Be sure to check The Journal Citation Report for "official" impact factors.
The Journal Impact Factor (JIF) is often considered to be the standard measure of the importance of a journal. The JIF is calculated on a yearly basis by dividing the total number of citations that an article in a journal received during the previous two years by the total number of citeable items that journal has published.
The JIF provides one specific number that can be used to easily compare the relative importance of journals within an academic field. However, the impact of a journal does not directly reflect on the impact of individual articles contained within it. For example, Nature reported that 89% of its citations were attributed to only 25% of its articles.
In addition, certain editorial policies can influence the impact of a journal. Journals may publish more review articles than case studies, for example, because the former are more likely to be cited more frequently. In some cases, editors have requested that authors add superfluous citations to articles previously published in the journal in an attempt to bolster the impact of that journal.
While any metric will likely have flaws, the JIF is widely used to illustrate the impact of a journal. In order to demonstrate the impact of one's particular publications, one will need to employ article- or author-level metrics.