The double arrow in the dissociation equation for a weak acid indicates a reversible reaction. This means that the reaction can proceed in both the forward and reverse directions, and that the reactants and products are in equilibrium with each other.

For example, the dissociation equation for acetic acid is:

$$\ce{CH3COOH <=> H+ + CH3COO-} $$

This equation shows that acetic acid can dissociate into hydrogen ions (H+) and acetate ions (CH3COO-). However, the reaction can also proceed in the reverse direction, where hydrogen ions and acetate ions recombine to form acetic acid.

The double arrow indicates that the concentrations of the reactants and products are constantly changing, and that the system is in a state of dynamic equilibrium. The position of equilibrium, which is determined by the equilibrium constant, dictates the extent to which the acid dissociates.