Existential instantiation is a fancy way of saying "creating something that wasn't there before". Imagine you have a toy box filled with all different kinds of toys. One day, you decide to make a new toy that isn't already in the box. You use your imagination and create a brand new toy out of nothing - this is like existential instantiation.
In the grown-up world, existential instantiation happens when we create or identify something that didn't exist before, like when we discover a new species of animal or find a new mathematical formula. It's a way for us to expand our knowledge and understanding of the world around us.