The impact factor is a measurement of how influential a journal is.
Researchers use it to determine how important an article might be or whether getting published in the journal carries any weight.
Quoting ISI (originators of the impact factor):
The JCR provides quantitative tools for ranking, evaluating, categorizing, and comparing journals. The impact factor is one of these; it is a measure of the frequency with which the "average article" in a journal has been cited in a particular year or period. The annual JCR impact factor is a ratio between citations and recent citable items published. Thus, the impact factor of a journal is calculated by dividing the number of current year citations to the source items published in that journal during the previous two years
Here's how you get the impact factor.Go to an ISI Citation Analysis database (different ones for different disciplines).
After you find a citation, look over to the right.
Which then takes you to this screen.