Bibliometrics are the traditional use of citation counts to analyze books and articles for how much they, and their authors, appear in the larger literature of their subject area. This measurement of impact and output can be used in promotion and tenure, as well as in attaining funding and grants.
The video below serves as a good, quick introduction to the idea behind this particular measurement.
This research guide will help you make decisions about which sources might work best as you work to determine journal impact, author impact, and article impact. It will also walk you through some history of the impact factor, what it can and cannot tell you, and efforts at reform.