Index are the data organizing mechanism which are similar to the database of relational database system.
Mapper is the system to map recommender key to data source key. Therefore solution can understand data from any data source generate intelligence from them.
Reindexing is a process to copy documents from one index to another, optionally filtering the source documents by a query, fetching the documents from a remote cluster. You will about to change the settings and mappings of a destination index with reindex.
Index settings are one of the index modules created per index and control all aspects of the index.
Index mapping is the process of defining how a document and the fields it contained are sorted and indexed.