It's a common prefix in many words related to the Earth, such as:

* Geography: The study of the Earth's surface, its features, and its inhabitants.

* Geology: The study of the Earth's physical structure and substance.

* Geophysics: The study of the Earth's physical properties and processes.

* Geodesy: The science of measuring the Earth's size and shape.

* Geochemistry: The study of the chemical composition of the Earth.

So, when you see the word "geo" in a word, it usually indicates that the word is related to the Earth in some way.