What is organic evolution?

Literally, it's the evolution of living things. Generally it's refered to simply as evolution.

The short answer is evolution is the gradual shift of living things through natural selection.

A simple way to visualise this is imagining a single creature and it's children. Obviously, the children don't look exactly like the parent (Variation), so one will be taller, one stronger, one faster, and so on. As those children grow up, the ones with an advantage do better. The faster child can outrun a predator and survive, the taller one can reach food the others couldn't, and so on. This advantage means the 'better' child is more likely to live long enough to breed.

For our example, we'll say the tall child has an advantage. Food is higher up, and he has an easier time getting to it. That means he spends less time gathering food, and more time trying to breed. The shorter ones spend their time trying to eat enough to survive, and don't get a chance to breed (or aren't strong enough to get a mate).

These grandchildren of the original will then look like their father who is slightly taller. Those children will also have variation... some will be shorter, and some will be even taller. Those even taller kids now have an advantage. And so on, and so on. Each family of the animal looks pretty much the same as it's parents but over the course of thousands and thousands of generations, the result can be totally different.

This example is a huge simplification of evolution, however it serves to convey the idea. If you are interested in Biology and evolution, please feel free to read any of the countless articles online about the topic in greater detail.