Determinate vs. Indeterminate.
A static system is determinate if it is possible to find the unknown reactions using the methods of statics, that is, by using equilibrium equations, otherwise it is indeterminate.
In order for a system to be determinate the number of unknown force and moment reaction components must be less than or equal to the number of independent equations of equilibrium available. Each equilibrium equation derives from a degree of freedom of the system, so there may be no more unknowns than degrees of freedom. This means that we can determine no more than three unknown reaction components in two-dimensional systems and no more than six in three-dimensional systems.
An indeterminate system with fewer reaction components than degrees of freedom is under-constrained and therefore unstable. On the other hand, if there are more reaction components than degrees of freedom, the system is both over-constrained and indeterminate. In terms of force and moment equations, there are more unknowns than equilibrium equations so they can’t all be determined. This is not to say that it is impossible to find all reaction force on an over-constrained system, just that you will not learn how to find them in this course.