What is native browser support that is utilised by webdriver?

[prateek goyal]

Hello,

I am new to selenium webdriver. As I was going through sleniumhq website, I came to know that:

"Selenium-WebDriver makes direct calls to the browser using each browser’s native support for automation."

(Selenium RC used Javascript libraries, that is pretty clear to me. and this libraries are injected in browser when we run selenium-RC.)

I want to know what is this native support being talked about for web driver? Is it the way browsers are coded? Or something related to JavaScript? 

I asked this question on stack-overflow also, but could not get a satisfactory answer. Please help.