What does subscript n mean in chemical formulas?

Have you heard of or studied polymers?  like polyethylene. == you start with ethylene,  C2H4, and cause the double bond in the molecule to break and many of these monomers attach forming a longer moleculeC2H4-C2H4-C2H4 ... different combinations of temp and pressure can stop the reaction with different chain lengths giving the plastic different properties.  

Kaolin is similar -- the clay is heated in a kiln, which does more than just dry out the wet clay, it also creates a chain type macro-molecular structure, making what we call 'china' plates, cups, etc. = = the 'n' just refers to the fact that this is many of the smaller molecules all attached to make this larger structure but the exact number is unknown and varies based on the timing of the entire process of heating, cooling, an moisture content after the heating.  Dr W's info is correct == I was just trying to add a more info for an explanation.

are you asking about this "n".... [Al₂(OH)₄Si₂O₄]n

if so.. that n means any integer.  The [Al ₂(OH)₄Si₂O₄] is a repeating unit.

.. for n=4... .[Al₂(OH)₄Si₂O₄] - [Al₂(OH)₄Si₂O₄] - [Al₂(OH)₄Si₂O₄] - [Al₂(OH)₄Si₂O₄]

etc