Plato's Forms are supposed to be the fundamental unchanging and eternal "spiritual" abstractions on which the mutable material world is constructed. It is easiest to understand geometrical forms, but the idea is that all that is at all real (such as "justice") is only real because it participates in the absolute reality of the forms.
The "Form of the Triangle" (or the "Ideal Triangle") is the perfection or definitive triangle. It is no actual triangle and does not have angles of any particular magnitudes; but it has three vertices and three perfectly straight sides. The Ideal Euclidean Triangle also exits in un-curved space and so its angles add up to precisely 180 degrees, even though they have no actual values.
The "Form of the Triangle" could be expressed exactly in terms of various mathematical formulae. However such an expression is not exactly the Form itself, because it has no significance in itself (after all it is no more than a set of black marks on white paper or one and zeroes in a computer's memory), but only the significance which the crafter of that expression intends in terms of the symbology of the (mathematical) language employed.
The Forms are not "empty". They are immaterial, though. It is matter (according to modern physics) that is "empty". The Forms and most especially "the Good" are full-of-being and omnipotent.
Augustine definitely understood "the Good" to be "God" and it seems to me that any monotheist has little choice but to do so - or perhaps "the good" is the "mind of God" rather than "God unqualified", I don't know.