This comes from a countdown convention used in by both the American military and NASA.
Generally, it is used when counting down to a major event that will happen at a specific time.
Mathematically, T is time, minus whatever amount of time is left until the event happens.
If the News Years ball is dropping in 10 minutes, one could say "The ball is dropping in T minus 10 minutes and counting!"
Therefore, "T minus zero" means that there is no time left.