You might see something that resembles a colourful doughnut with a score in its centre when looking at some articles in ProQuest or elsewhere. This is an Altmetric donut, a visualisation "designed to help you easily identify how much and what type of attention a research output has received". It is just one way of presenting altmetric data.
What are altmetrics?
Altmetrics are an alternative to traditional bibliometrics that measure citation impact. Citation impact involves statistical analysis of how often, say, an article was cited in other articles and books. By looking at how often an article has been cited you can get an indication of its diffusion throughout scholarly research. Decisions on tenure, grants and funding for research can often take this type of information into account.
Altmetrics can compliment citation impact measurements. They look at how often articles are cited in non-traditional sources such as news, blogs, shares on social networks or use in reference management tools. They can be used to look at how often an article has been viewed, saved, cited or discussed.
Altmetrics can be useful indicators but you should bear in mind that they are only indicators and you should approach them critically and not take high scores as confirmation of quality or accuracy. Konkiel, Sugimoto and Williams make a good point in their blog post 'What constitutes valuable scholarship? The use of altmetrics in promotion and tenure': "A hundred thousand tweets about a paper on HIV may be a signal of attention; the fact that many people have read the paper and have changed their personal practices evidences impact. In short, we must take care not to mistake attention for impact."
Image credit: https://www.flickr.com/photos/javcon117/17426787281 (CC BY-SA 2.0)