Journal metrics measure the number of times articles from a specific journal have been cited by other articles. There are many different types of journal metrics and each organization count citations in slightly different ways.
Journal metrics are used as a means of comparison that demonstrate "impact" in the tenure or job search process.
However, its important to note there are several other types of metrics that can also be used to demonstrate impact:
Choose the metrics that give the best context for the impact of your work.