Terrestrial Time (TT) is a Time Standard designed for describing astronomical observations from Earth, that allows simple arithmetic to determine time intervals, precise to the millisecond level. Certain observations require that time precision, e.g., observations of moving planets and stars and observations of Pulsar pulses and glitches. The J2000.0, B1950.0, and B1900.0 are defined in terms of Terrestrial Time.
Such a time standard is non-trivial and Terrestrial Time is the latest standard by the International Astronomical Union to provide one. Other time standards are also used, e.g., to compensate for Earth's orbit: light from a nearby star viewed from Earth will reach Earth earlier by minutes if Earth is currently at the point of its orbit closer to the star.
Terrestrial Time is based on the SI second, and a point in time was chosen when it matches other standards, but it "drifts" from other standards.