A stable truss when the loads are applied, it maintains in its position and configuration. But when the loads are removed it easily comes back to the first, original position. Unstable truss in other hand, has undergoes rigid body motion. That means when the loads are removed this truss don’t come back to the original position. So, it does not satisfy the equilibrium position in every loading case.
The video bellow will explain more about the calculations and all kinds of stability and determinacy in trusses.

Source.

...