什么是XMLHttpRequest？

I was reading about Jasmine over here , which states:

When you want to mock out all ajax calls across an entire suite, use install() in a beforeEach. Because jasmine-ajax stubs out the global XMLHttpRequest for the page, you’ll want to uninstall() in an afterEach so specs or setup that expect to make a real ajax request can.

I'm relatively new to js side, I wonder whats exactly XMLHttpRequest is?

If I'm doing something like this in jquery:

$.ajax({
  url: "test.html",
  context: document.body
}).done(function() {
  $( this ).addClass( "done" );
});

then how come $.ajax is related with XMLHttpRequest. And they meant by global XMLHttpRequest?