Altmetrics measure the influence an article has by analyzing citations and sharing activity. The higher the altmetrics score, the wider the dissemination of the work.
Altmetrics are basically counts of mentions and links to scholarly journals and datasets from reputable news sites (and aggregators) and blogs, peer-reviewed sites, reference managers, and major social media sites.
The company Altmetric uses a weighted score with a news article counting 8 points, a blog post counts 5, a Wikipedia link is 3, and a Twitter post counts as 1 point. Sources counting less than 1 point include Facebook and YouTube at 0.25 point and LinkedIn at 0.5.
Other altmetrics sources use a simple count of "mentions" with no weighting applied. Score from different sources aren't directly comparable, but a higher score generally means a more influential scholarly work.