For Kierkegaard, the main antecedent of existentialism, existence is above all an existing one: the existing human one. It is the one whose 'being' consists of subjectivity, in pure freedom of choice. One cannot speak of the essence of existence, one can only speak of 'this existing' or 'that existing' and the truth of these is nothing but 'subjectivity'.
Thus, for Kierkegaard, existence is tantamount to making a 'final decision' regarding the absolute divine transcendence and such a descent determines' the moment 'which is neither the mere creep of' universal time 'nor any participation in an eternal intelligible world. In this context, philosophy is not speculation but 'decision' insofar as it is an affirmation of stocks.
This priority that Kierkegaard has to exist over essence will be common to many other contemporary authors such as Nietzsche, Dilthey, Bergson, Sartreand Heidegger, even though they all started from different assumptions.
Central Concepts of Existentialism
Sören Kierkegaard, the father of existentialism
Heidegger and the failure of Western metaphysics
Sartre and French Existentialism