Mapping is the process of defining how a document should be mapped to the Search Engine, including its searchable characteristics. ElasticSearch allows for support of multiple typed mapping definitions registered with the same index, and different types registered with different indices.
Explicit mapping is defined on an index type level. By default, there isn’t a need to define explicit mappings as they are automatically created and registered once new type or new field is introduced (with no performance overhead) and have sensible defaults. Only when the defaults need to be overridden then mappings should be set.