What's the condition for a system to be stable. What will happen if it gets unstable?

For a system to be stable, the condition is that the output given by the system is a controlled output. If the system becomes unstable, its output goes out of control.

Explanation:

To govern the stability of a system, certain parameters are put into place. The most reliable parameter out of this is that the system is giving a desired and controlled output. Both, an increased or a decreased output is the primary indicator of the instability of the system.