The word "terrestrial" refers to the Earth or its inhabitants.
Therefore, extraterrestrial means "beyond the Earth" or "originating from outside the Earth".
It's often used to describe:
* Life forms from other planets: We might talk about searching for extraterrestrial life.
* Objects or phenomena originating outside Earth: We could discuss extraterrestrial meteorites or the effects of extraterrestrial radiation.
In essence, "extraterrestrial" signifies something that is not from Earth.