Boosting is the process of enhancing the relevancy of a document or field. Field level mapping allows to define explicit boost level on a specific field. The boost field mapping (applied on the root object) allows to define a boost field mapping where its content will control the boost level of the document. For example, consider the following mapping:

{
    "tweet" : {
        "_boost" : {"name" : "my_boost", "null_value" : 1.0}
    }
}

The above mapping defines mapping for a field named my_boost. If the my_boost field exists within the JSON document indexed, its value will control the boost level of the document indexed. For example, the following JSON document will be indexed with a boost value of 2.2:

{
    "my_boost" : 2.2,
    "message" : "This is a tweet!"
}