why is the phrase "t minus " used before a shuttle liftoff?

Answer 1

As I told you in your other question, the space mission time clock is based on the time after the start of the mission, which is defined as lift off, thus you count down to lift off (T-minus) and you count up after lift off, until the mission ends when they arrive back on Earth.

Answer 2

T = time to lift off.

T Minus is how much time until lift off.

T Minus 1 hour means one hour until lift off.

T Minus 10 seconds means 10 seconds till lift off.