what does chaining do in javascript code example

Example: method chaining

given() ....   --> RequestSpecification 
     .header  accept() contentType()
     .queryParam
     .pathParam
     .body
     .log
     .auth..
when()       --->> RequestSender
   .get() ------>> Response Object 
   .post()
   .put()
   .delete()
then()      -----> ValidatableResponse  
        This is where assertion happen 
    .statusCode
    .header   accept() contentType()
    .body( matchers goes here)
    .log