The Kirchoff Voltage law states that the sum of emfs in a circuit is equal to the total potential drop in the circuit.
So for a simple example, where you a 6V cell, for example, and 2 resistors in series.
The 6V cell can be seen as a place where the water is given potential energy - if we imagine a ramp, it would be the water being pumped up to the top of the ramp.
The water then flows along a straight path (wires assuming 0 resistance) until it meets a resistor. A resistor can be seen as another ramp, however since there are 2 resistors, (assume they are the same resistance, however this doesn't really matter) all the potential cannot be dropped across one resistor so this ramp is smaller than the ramp up from the 6V battery. As water falls down this ramp, it loses potential energy - this is 'the same' as the drop in potential difference across a resistor.
You may ask - but the water speeds up when it falls down the ramp? well for this we should assume that there is no gain in kinetic energy and all gained kinetic energy is dissipated falling down the ramp - this can be analogous to current remaining constant in a series circuit.