Journal metrics are an attempt to quantify the importance of a particular journal in its field. Providers of journal metrics use some type of formula that takes into account the number of articles published per year and the number of citations to articles provided in the journal.
Journal metrics can be used to help quantify and determine how your research is being used, identify like researches and institutions for potential collabortion, justify requests for grants and other funding, and support your applications for tenure and promotion.
Use of journal metrics should be used with caution as they only tell one part of a larger story of the true impact of research. Journal metrics should be used in conjunction with other ways to evaluate research, researchers, and publications.