Mapping is the process of defining the schema or the structure of the documents. It describes the properties of the fields in the document. The properties of the field include the data type (for example, string, integer, and so on) and the metadata. In the previous chapters, we discussed how the documents are converted into the inverted index when we index them. During indexing, the mappings of the fields define how the fields are indexed and stored in the inverted index.
Just like you would define table schema in SQL, it is important to set the mappings of the index before you index any data. As we discussed before, a type in Elasticsearch is like an SQL table, which groups documents of similar nature (you would define one type for users, one for orders). Each type has its mapping defined. Having different mappings could also be a motivation to define a new type.