Journal impact measures the influence or importance of a particular journal in a discipline. Journal impact metrics take into account the number of articles published per time period and the number of citations to articles published in that journal. They can help track citation patterns within journals and determine which journals are highly cited.
There are many factors that influence the impact of a journal and each tool may produce varying results depending on the scope of its database. For a more accurate picture of journal impact, you can use more than one tool to compare results.