Altmetrics can encompass many things, but simply put it refers to 'alternative metrics' on the article level. It tracks non-traditional engagement with articles and showcases attention to research outside of academia. It includes how many times an article has been bookmarked on social media, how many times it’s been mentioned, how many people have blogged about it, any kind of print news coverage it's received, how many people have opened it, and how many downloads it’s gotten. It provides immediate feedback on attention to your research; it can be particularly useful for early career researchers that don't have many citations.
Altmetrics complement traditional citation based metrics and can provide you with a fuller picture of your research impact.
Altmetric (singular, with a capital A) is a company that provides software for tracking an article's altmetrics; the article needs a DOI to be analyzed. The Altmetric Bookmarklet is a free tool that can be added to the bookmarks on your browser to easily see an article's engagement.
PlumX is a tool available in Scopus that provides altmerics data for published articles. It categorizes engagement into five groups, shown in a 'Plum Print'--a data visualization method. The size of each color corresponds to the amount of engagement for that category.
It is accessible under the 'Metrics' tab when viewing an article on Scopus.
Similar to bibliometrics, altmetrics have limitations.