According to Wikipedia:
“Altmetrics are new metrics proposed as an alternative to the widely used journal impact factor and personal citation indices like the h-index.”
You can see them looking like a colorful “donut” at the left of the document title in the main list.
If you tap on this donut, or if you swipe to the left this paper and select the “More” menu, or from the metadata view, you can get more details about the Altmetric for the selected paper.
Altmetrics are provided by Altmetric.com.
Currently they are mainly available for biology research articles.