When a visitor views one of your pages which contains a media, the time time when the page was loaded is measured.
When a visitor then plays a media on your page, Media Analytics processes how many seconds have elapsed since the page was loaded by this user. The difference is the “Time to initial play”. For example if a visitor opens a webpage at “12:25:35” and starts playing the video at “12:25:41”, the time to initial play is 6 seconds.

Previous FAQ: What is a media “impression” and “impression rate”?