Loading time — time from the beginning until the end of the page downloading (onload event). For browser that do not support Navigation Timing API, as well as for statistics before 14 July 2014 inclusive, it is a time from the beginning of the counter code execution to the end of the page downloading (onload event).
This is important performance indicator of your website; it shows how quickly pages are opened. The smaller this time, the less likely that the user will go away from your website without waiting for result.