For configuration see Configuring - Storage Plugins.
A storage backend is a "Storage" plugin, which defines how to list, create, update, delete and retrieve file contents and metadata.
Rundeck bundles two storage plugin types:
file- stores file contents on disk
db- stores file contents in the database
A storage converter is a "StorageConverter" plugin, which can modify the contents or metadata of a stored file as it is being created, updated, or read.