For many centuries, philosophers attempted to discover and describe the attributes of human nature: rational thought, imagining the future, using language. The essence of a person, the who-we-are, precedes our existence in the world.
Existentialists turned philosophy upside down by arguing that, except for biological necessities, there is no human nature to be discovered, no universal truth across people. Within the confines of our time and place of birth, we become who we choose to be.
Unlike a dog whose behavior is recognized in any place or time, human behavior is vastly unique. We can live as ancient nomads in the desert or artists in a modern city, become a monk isolated from the world or a pop star on the stages of the world, hold radical political views or be completely ignorant of our surroundings.
No one can look at an infant and determined who this child can become nor how it will behave. There is no biological necessity to be or act in any particular manner. We exist and then we become, or or existence precedes our essence.