Which effect does adding hydrogen chloride have when determining the concentration of sodium hypochlorite in bleach via titration have? Many of you might be familiar with this and I do understand this completely, however I do have a question. 

If you are familiar with this, then this is the equation and this is the question for it. 

$$\ce{ClO^{-} ~(aq) + 2I^{-} ~(aq) + H2O ~(l) -&gt; Cl- ~(aq) + I2 ~(aq) + 2 {}^{-}OH ~ (aq)}$$

So for the equation above I understand everything except that in the titration process my instructions say to add $\ce{HCl}$ into the solution, I do not understand how adding $\ce{HCl}$ will have any impact.

I will provide an accurate reference later.
The reaction is in equilibrium and if you don't add any acid, then some of the $\ce {ClO-}$ does not react. Adding acid will change the equilibrium to the right so most of the $\ce {ClO-}$ reacts.