It's used in a variety of words to indicate something tiny or minute, such as:
* microscope: an instrument for viewing tiny objects
* microorganism: a living organism too small to be seen with the naked eye
* microwave: a type of electromagnetic radiation with a short wavelength
* microcosm: a small world or system that is considered to be representative of a larger one
The word "micro" is often combined with other words to create new terms, like "microbiology" (the study of microorganisms) and "microchip" (a tiny integrated circuit).