The search index contains #[doccount]# documents. You can delete them here.
Deletions are made concurrently which can cause that recently deleted documents are not yet reflected in the document count.
Index deletion will not immediately reduce the storage size on disk because entries are only marked as deleted in a first step.
The storage size will later on shrink by itself if new documents are indexed or you can force a shrinking by performing an "Optimize Solr" procedure.