What is the 10% rule? what is its significance? why is energy lost?

The 10% rule means that approximately there will be only 10% energy transferred from a trophic level into another. That was because the consumer on the upper level will use the energy before it was consumed. About 90% of the energy used so only 10% remain will be transferred.
Example: an antler eating grass with a total 100 calories. Then the antler is moving and growing, using 90 calories. Then a lion eats the antler, but the energy remains is only 10 calorie because 90 calories are already used.

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.