What Do the Brackets in the Following Equation Represent? [H₃O⁺]

What do the brackets in the following equation represent? [H₃O⁺] × [OH^-] = K_w

A) The brackets mean the molarity of the compound inside them.
B) The molecule inside does not react with the other molecules in brackets.
C) The molecule in brackets is undergoing hydrogen bonding with the other molecule in brackets.
D) The first molecule in brackets is an acid and it reacts with the second molecule in brackets,which is a base.
E) none of the above