XMLHttpRequest (XHR) is an API in the form of an object whose methods transfer data between a web browser and a web server.
XMLHttpRequest object is used in AJAX model to communicate with servers.
The idea originated in Microsoft and was first implemented in Internet Explorer 5 around 1999. Later it was standardized and adopted in all browsers by 2006.
Google Maps was one of the first adopters of this model showing the business value you can get in the products.
Browser support of XHR was not consistent at that time plus handling of various response types and errors required some extra coding. But it pushed the industry forward.