Altmetric is a system that tracks the attention that research outputs such as scholarly articles and data sets receive online. It pulls data from:
Altmetric cleans up and normalizes the data from these sources then makes it available for analysis. A key difference between Altmetric and other social media monitoring services is that Altmetric will disambiguate links to outputs: it knows that even though some X posts might link to a PubMed abstract, newspapers to the publisher's site and blog posts to a dx.doi.org link they're all talking about the same paper.
After Altmetric aggregates all of the information (we call each piece of information a post) it can find about a research output it looks at both the quantity and the quality of attention being paid to an output and visualises it.
For more information about the product, check out the Library's Dimensions and Altmetric CenterNet page, or visit the Altmetric product site or support portal.