Sartre, along with others such as Kierkegaard, Jaspers, Heidegger, Binswanger, Camus, articulated a philosophical view of life focused on the very fact of our existence -- hence "existentialism"
The four foundational ideas in existential philosophy are finitude, autonomy, responsibility, and authenticity.
Finitude recognizes that we are finite, that we are going to die and that typically we're not too happy about this prospect. We might find ourselves in denial of our own own eventual death and thereby limit our capacity to live fully in this moment. The saying goes, "we refuse the loan of life for fear of paying the debt of death."
Autonomy recognizes and invites us to be free -- free from the arbitrary influences of others and culture, free to be our own individual selves and to pursue our unique values, desires, and visions. It takes work to be autonomous, to break ourselves free from what Nietzsche called the "herd" mentality. It's just as hard for us today to figure out who we are and what we want when we are inundated with commercial messages and cultural images