a spring: a place where fresh water rises up out of the ground
A spring is a place where fresh water rises out of the ground on its own. In a dry land this was the difference between life and death, so a dependable spring shaped where people settled, which routes they traveled, and even how they fought: an army would stop up an enemy's springs to force a surrender.
Most of the time the word simply names such a water source, whether an ordinary hillside spring, the named spring that marked a boundary between tribes, or the springs God is pictured opening and shutting in creation and in the flood. From that plain picture the writers build some of their richest images. A bride is called a spring locked and a fountain sealed, belonging to her husband alone. Salvation becomes a set of wells from which people draw water with joy. Zion is the source that every song traces back to.
Through all of these the core idea never changes: living water that flows up freely and gives life. That is exactly why the word works as a picture. A spring is close kin to a dug בְּאֵר, "well," and to מָקוֹר, another word for a source, but a spring is specifically water that comes up by itself rather than water reached by digging.