Let's take the example of copper sulfate penta-hydrate: $\ce{CuSO4. 5H2O}$.  The dot here is used essentially as an expression of
ignorance to indicate that, though the parts of the
molecule separated by the dot are bonded to one another in some fashion, the exact structural details of that interaction are not fully expressed in the resulting
formula.

Using Alfred Werner’s coordination
theory that indicates
that four of the five water molecules are actually
bonded directly to the $\ce{Cu}$ in the form of a complex
aquo ion  $\ce{[Cu(H2O)4]^2+}$, then the formula is better
expressed
as  $\ce{[Cu(H2O)4][SO4].H2O}$

So, the dot here signifies ignorance of how the
subunits of a formula are structurally related.

Now, let's