This is the time from when the user started navigating to the page until when the DNS servers for the website have confirmed resolution of the DNS for that domain.
The time following DNS resolution up to the point at which the TCP/IP connection from the browser to the resolving server has been confirmed
The First Byte time is the time from establishment of the TCP/Ip connection until when the first bit of the server response arrived. The bulk of this time is usually referred to the "back-end time" and is the amount of time the server spent building the page for the user.
The period between first byte and when the Browser commences download of the page and/or page objects.