Journal citation metrics are intended to help scholars identify highly cited journals in their field. Citation metrics measure how frequently publications have been cited with the goal of measuring the impact of the journal within that discipline. Keep in mind, this is not a direct measure of the quality of the publications. There are many different metrics available, each with its own formula for measuring impact. As a result, you may want to consider looking at a variety of metrics. Some things to consider when you are using citation metrics:
Altmetrics are complementary to traditional, citation-based metrics. They typically track mentions to scholarly research in social media, the news, blogs, etc.
Impact Factor: The Measure of a Journal - Randy Souther, Gleeson librarian and editor of the open access journal Bearing Witness: Joyce Carol Oates Studies, discusses Impact Factor and alternative journal metrics.