The multiplier, also known as global multiplier, increases your income. Multipliers are cumulative, meaning when getting additional ones they do not multiply, but add to each other.

For example, if you have a multiply of x5 and you get a multiply of x10, your final multiplier adds x15 to your income. It acts as an additional multiplier for the company profit generated by all the girls. Take into account that Prestige takes effect even if a multiplier is not active.

You can check your global multiplier and many other stats in the settings.