The Load Time is measured as the time from the start of the initial navigation until the beginning of the window load event (onload).
In the above definition what does initial navigation and window load event mean?
can someone please elaborate
Thanks 
Initial navigation is the point where WebPageTest tells the browser to load the given page.
The load event is for a document is specified here: HTML Standard
Basically for most pages it is when the full HTML has been loaded, all of the scripts have been executed and all of the images and resources referenced by the HTML have loaded.