Journal impact factor is the way journals are measured in terms of quality and are a good way to select journals that you are thinking of publishing in. It measures the amount of citations a journal has had over the previous year. It is calculated using the formula:
Number of citations received in 2019 by items published in the journal in 2017-2018.
Number of citable items published in the journal in 2014-2015
In essence it measures the amount of citation the items in the journal have had during the current year in order to showcase quality.
Check out the tools below that allow you to calculate journal impact factor.
However journal impact factor is not a completely comprehensive measure and can be skewed as seen in the video below.
So remember to do in-depth research into each journal where you are thinking of submitting work.