Conservation of energy
Energy is never created or destroyed. It changes form, and the total stays the same in a closed system.
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Analogy
Moving money between pockets does not change your total.
Example
m g h = 1/2 m v^2v = sqrt(2 x 9.8 x 10)v = 14 m/s
In practice
A ball dropped from 10 m hits the ground at about 14 m/s, ignoring air.
Questions people ask
Where does energy go when friction acts?
It turns into thermal energy, which warms the surfaces and the air a little. The total energy is still conserved, but the mechanical part (kinetic plus potential) shrinks. This is why rubbing your hands warms them and why a rolling ball slows down.
Is a perpetual motion machine possible?
No. Real machines always lose some energy as heat, so they cannot keep running forever without an energy source. A machine that made extra energy would break the conservation law, which has passed every experiment so far. Such claims always hide a flaw or a hidden battery.