Elasticsearch mapping is a crucial feature that defines how documents are indexed and stored in Elasticsearch, essentially acting as a schema for JSON documents. While Elasticsearch can function without explicitly defined schemas, known as being schema-less, defining custom mappings is advisable to avoid issues such as incorrect type detection and unnecessary data duplication. Mappings are structured hierarchically, starting with a root level that specifies field properties and settings, and can be provided either during index creation or via the Put Mapping API, although the former is generally recommended to avoid conflicts. The mapping allows for detailed configuration of document fields, including setting types for fields at various hierarchical levels, with Elasticsearch supporting a variety of built-in types like geo_point and ip for specialized indexing and search tasks.