Fusion collections are Solr collections that are managed by Fusion. Solr itself manages a set of resources for a collection. Stopword lists are one such resource.
Stopwords are words that are filtered out of a user-entered search request. Examples of common English stopwords are a, an, and the.
The Fusion UI provides a stopwords manager tool that is reached from the Stopwords page in the Collections menu. This tool lets you view the contents of all configured stopwords files in an editable browser.