The square brackets are used in case of complex compounds. This is known as coordination sphere. There is a metal in the coordination sphere and ligands attached to it. This complete set of metals and ligands inside the brackets is complex ion or molecule.
The ions outside this coordination sphere are counter ions of the complex.
It represents that if the complex compound is dissolved in water it will give a complex ion and the counter ion. The atoms in the bracket cannot break there bonds and will remain un-dissociated.
If there is no such bracket then it will represent simple or binary salt.