electrical power
Power is how fast energy is being used or delivered, the rate of work. A bright 100 W lamp burns energy faster than a dim 5 W night-light; a powerful motor does more work each second than a weak one. In the water picture, power depends on both how hard the water pushes (pressure, voltage) and how much is flowing (flow rate, current). A trickle under huge pressure, or a flood under tiny pressure, can each carry little power; you need both.
For electricity the rule is beautifully simple: power equals voltage times current, P = V times I. Voltage in volts times current in amps gives power in watts (symbol W). A 12 V car bulb drawing 2 A burns 12 times 2 = 24 W. Because voltage, current and resistance are tied by Ohm's law, you can also write power as P = I^2 times R or P = V^2/R, handy when you know the resistor and one of the other two.
Power matters because energy that goes into a component has to come out somewhere, usually as heat. A resistor turning 0.25 W into heat gets warm; push 2 W into the same tiny resistor and it scorches, which is why every component has a power rating you must respect. Power tells you battery life, how big a heat sink you need, and whether a wire will glow. It is one of the most practical numbers in all of electronics.
A resistor passing 0.1 A with 5 V across it dissipates P = 5 times 0.1 = 0.5 W, so a common 0.25 W resistor would overheat; you would choose a 1 W part.
Always check a part's power against its rating before trusting it.
Watts are not the same as volts or amps; a low voltage can still mean high power if the current is large, which is exactly how a 5 V, 20 A supply can deliver 100 W and melt a thin wire.