Biological evolution is defined as organisms reproducing but experiencing changes with each generation. Evolution can happen in a small and large context. There are small genetic changes between generations, as well as large changes that happen over multiple generations.
In order for a change to be defined as biological evolution, it has to occur because of genetic influences. For example, a tree losing its leaves is not biological evolution, but if those leaves turn a darker shade of green over multiple generations it is.
One theory underpinning biological evolution is that all species descended from one common ancestor. They eventually grew to separate into different species as they adapted to their respective lifestyles and environments. One example of a species adapting to its environment and lifestyle is humans developing a bipedal stance. Prior to walking upright, humans moved around like apes, which meant they used their hands as well as their legs. When the need to spend less time climbing trees and more time evading predators arrived, humans gradually evolved to walk upright. This meant the curvature of their spines changed and their hips narrowed. Bipedalism also represents an evolutionary trade off, as a narrower pelvis made it more difficult for females to give birth.