Facets are categories of information about search results. They appear on the lefthand side of the search results page and you can click them to narrow down your search results.
If the index you're searching allows it, you can view a facet as a table, pie chart, or bar chart.
For example, here is a pie chart for a facet that shows the different types of files returned by a search:
To view facet visualizations, click the chart icon for a facet.