FieldCollapsing is an Solr Feature to group Results coming from Sol.
From Solr docs: Field Collapsing and Result Grouping are different ways to think about the same Solr feature.
Field Collapsing collapses a group of results with the same field value down to a single (or fixed number) of entries. For example, most search engines such as Google collapse on site so only one or two entries are shown, along with a link to click to see more results from that site. Field collapsing can also be used to suppress duplicate documents.
Result Grouping groups documents with a common field value into groups, returning the top documents per group, and the top groups based on what documents are in the groups. One example is a search at Best Buy for a common term such as DVD, that shows the top 3 results for each category ("TVs & Video","Movies","Computers", etc)
https://cwiki.apache.org/confluence/display/solr/FieldCollapsing
