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 »?