Amphimixis is when two very special cells called gametes come together to make a new baby. Gametes are like tiny seeds that can only make a baby when they join together - like how you need both the seed from a mommy plant and a daddy plant to make a new baby plant.
One of the gametes comes from a mommy, and one comes from a daddy. The mommy's gamete is called an egg and the daddy's gamete is called a sperm. The egg and sperm join together inside the mommy's body to make a baby.
When the egg and sperm join together, they mix up their information. This information is called genes and it's what makes you who you are - like what color eyes you have or how tall you are. So when the egg and sperm come together, they make a new unique combination of genes that makes a unique baby.
Amphimixis is a very important process because it helps to create diversity in the world. If every baby was exactly the same as their parents, it would be a boring world - like if every apple on a tree was exactly the same, it wouldn't be as fun to eat them!