Delta time in F1 is the difference between two laps. Delta means difference and delta time means time difference. Usually, it’s the time difference between the pre-determined lap time and the current lap time.
Delta positive is when the driver is going faster than the predetermined lap time and being asked to slow down for safety. Delta zero is when there is no difference in time during two laps. This is an extremely rare phenomenon.
When the driver is going slower than the predetermined lap time, it is called delta negative. Delta time can be calculated between two different cars as well.