Altmetrics are measures of research impact that supplement citations. They measure the wider, societal impacts of scholarly works by tracking how they are discussed, shared, saved, read, and reused by scholars and the public. Altmetrics are sourced from many web platforms and medias: number of downloads, shares, and mentions in venues such as institutional repositories, government documents, social media (Twitter, Facebook, Reddit, blogs, etc), and web-based reference managers like Mendeley and Zotero.
They are useful supplementary measures of impact, best used in tandem with traditional measures like citation analysis. Combined, these metrics can illustrate the broad impact of your work.