You use T minus [time] to denote a countdown to the time an event occurs. You use T plus [time] to track the elapsed time since that event. This is language you might hear in a technical setting. Wikipedia gives the example of a rocket launch:
After a launch, most countdown clocks begin to show Mission Elapsed Time, which is typically shown as "T plus." The picture to the right shows "+ 00:00:07", approximately seven seconds after liftoff.
In the video, the man is explaining the events that occurred after the Big Bang. He called the time of the Big Bang "time zero". At T plus twenty minutes , he means twenty minutes after the Big Bang.