Can you explain the energy transformations that occur when moving from one trophic level to the next in an energy pyramid?

The transfer of energy from one thing to another thing is, in practice, a very inefficient system.

In the energy pyramid, we start with the Sun as it shines down on plants - let's say in this case plankton. Most of the energy that shines down onto the plankton is lost and unused (it's heat or reflected away or otherwise unused) but roughly 10% of the energy is used by the plankton to grow bigger and make more plankton. See this Socratic answer on the 10% rule.

Along comes a small fish to eat the plankton. The small fish can only use a small percentage of the energy contained in the plankton - there's the energy of digestion (and digestion is full of inefficiencies so that we don't get the most from it) plus a lot of energy of plankton was in building cell walls and other structures that the fish can't make use of. And so the small fish gets roughly 10% of the energy of the plankton into itself.

And along comes a large fish to eat the small fish. Just like in the other two examples, the big fish can only get a small percentage of the energy stored in the small fish (again, digestion is very inefficient and the small fish has many things like scales and bones that the big fish's digestion can't use). The big fish gets roughly 10% of the energy stored in the small fish.

And now an eagle swoops down to catch and eat the big fish and it's the same thing all over again - the eagle can only get some much use from the stored energy in the big fish.

Energy transfers in a food pyramid are pretty inefficient, which greatly limits the number of levels and the sizes of those levels.

The reason the transfers are so inefficient is because much of the energy of one level is used to keep the organisms living (roughly 90%) while the remainder is used to grow and create biomass (10%).

https://en.wikipedia.org/wiki/Ecological_pyramid