Here is my layman's understanding. You inherit dna (genes) from your parents which are like progams that tell your body how to grow. You might take all your genes from your father, or all from your mother, it's random so 'usually' about half and half. But you also carry and inherit a duplicate copy of all genes, which could be different, but your body doesn't use. These are dormant but you can still pass on. So e.g. eye color uses two genes, each parent has 4, you take 4 randomly and then randomly pick 2 for your eye colour. Some genes are dominant. Which means the choice is not so random e.g. if one your parents is a nog their brown eye trait is dominant so hybrids will nearly always get brown eyes. Cosmic radiation from space damages the dna in genes, (no really it's true). Some of the code gets fucked up, but only a little. Just enough to make you a little different from your parents. Your hair is a bit blonder, you are 1mm taller or shorter etc. Who gets to reproduce most passes on most of their genes. Poplulations mix, alot, so useful genes propagate widey across populations quite quickly (except for shitskins who marry cousins). Who gets to reproduce most is determined by the environment. Usually this is a very subtle process that takes thousands of years determined mostly by who has slightly more offspring on average. Think of a owl, cosmic ray alters a feather gene making it 10% softer. Owl kids make a bit less noise flying so catch more mice. More quieter owls survive better, have more chicks, pass on their new genes which spread. But with humans doing tribal genocides, birth control, infertility etc, I'm not sure you can say humans are evolving anymore? Even if you accept it's a blunt instument that can go backwards. Changes to whole populations within two or three generations is not evolution.