It's often used in medical terms to indicate a condition that is below normal or deficient, for example:
* Hypoglycemia: Low blood sugar
* Hypothyroidism: Underactive thyroid gland
* Hypotension: Low blood pressure
It can also be used in other contexts, such as:
* Hypocenter: The point beneath the Earth's surface where an earthquake originates
* Hypocrite: Someone who pretends to be something they are not
The prefix "hypo" essentially conveys the idea of something being lower or less than the norm.