I can't see the supposed chemical equations, but I'm guessing it's like every other chemical equations.
Now, the 2 added to the front of HCl is just to balance the chemical equation. It is specifying the amount of moles of HCl that is used in the particular reaction.
Essentially, the chemical equation is saying that 2 moles of HCl is reacting with however many moles of the other reactant to give the product you're seeing.
