The drop in electromotive force across each resistor hooked in parallel must be equal to the increase in electromotive force across the power source.
Voltage is a measurement of the electric potential difference across something that has been placed in a circuit. The voltage of a cell (or battery of cells) measures how much it increases the electromotive force; as current flows through a resistor, an equal drop in electromotive force occurs. In other words, however much the battery boosts the voltage, the resistors drop it by the same amount - hence, the need for the boost at the battery.
Let's use the following diagram as an example:
If the resistor slows the current by reducing the EMF by 12V, then the power source must boost the EMF back to its original strength by increasing by 12V.
An excellent tutorial on this can be found at:
An easy way to think about this is that if you are touching ONLY a wire, the voltage is the same along that wire, so from the bottom of the batter all the way to the bottom of the resistors, the voltage is 0. From the top of the battery to the top of the resistors, the voltage is the same (maybe 12V or something), so the "drop" across the resistors would be 12V for each.