Using resources available through the Countway Library along with other things that are available online, you can gain a sense of the impact of your scholarship, find out who's citing you, and use software along with other tools to craft a compelling narrative about your scholarship. The number of times you and your works (including publications, posters, data, code, etc.) have been cited is just one indicator of your impact.
Note that there is not one single metric that tells you the true impact of your research. Rather, you should look at a variety of different measures to gain a sense of your scholarship's influence.
Even if you don't believe that any individual metric (or set of metrics) can adequately communicate your scholarship's impact, they are used to evaluate researchers, groups, institutions, and even entire countries. They also factor into decisions regarding hiring, promotion, tenure, and funding.
Metrics cannot tell you anything about the quality of your research. They are not direct measures of impact, quality, or performance; rather, they are indirect measures that look at quantitative indicators of "attention" (citation counts, page views, downloads, etc.) and at best, they can be used as proxies to help you tell the story you would like to tell about your scholarship.