The impact factor of an academic journal is a measure reflecting the average number of citations to recent articles published in that specific journal. It is often used as a metric for the relative importance and/or impact of a journal within its field. Journals with higher impact factors are therefore noted to be more important and influential than those with lower impact factors.
The journal Impact Factor is the average number of times articles from the journal published in the past two years have been cited in the Journal Citation Reports (JCR) year. Journal Impact Factors are released annually by Clarivate Analytics' Journal Citation Reports.
Google Scholar's Metrics will tell you about impact factors among other bibliometric information. On the main page of Metrics, you can search by field or specific journal. There is also a listing of top journals based on the h5-index and median.
Impact factors can be used to identify journals that are relevant to your research and where you may want to publish your work. Furthermore, you can confirm the status of a journal that you have already published your work.
But what does the number actually tell us? To say that the British Medical Journal (BMJ) has an impact factor of 9.052 is fairly meaningless. It makes more sense to say that BMJ’s impact factor ranks 6 of 105 journals in its field. Or to compare the journal’s impact factor of 9.052 with the average impact factor for its field: 4.326. So, remember that the impact factor for a journal should not be looked at in isolation. Rather, the impact factor of a journal should be compared to the impact factors for other journals within the same subject category for context.
Other things to consider about Impact Factors: