What does Sartre mean with that existence precedes essence and how is it related to the earlier existential philosophers' thoughts?
Existential philosophers like Kirkegaard, Heideggers and Friedrich Nietzsche?
Existence precedes essence means human beings are not defined by an a priori nature. Rather, what's fundamental about us is that we exist. This stands in contrast to traditional philosophies and theologies which claimed people had an essence (e.g. man is a reasoning animal, man has a divine spark, etc...).
Bare existence lacks meaning, as such we are faced with dread, awe, and to believe in something greater requires a leap of faith (Kierkegaard), for we don't have the comfort of "knowing" what to do. Additionally, the lack of meaning invites nihilism, atheism, iconoclasm and of course freedom and its implications on morality, and this is where Nietzsche comes in.
Furthermore, bare existence is about consciousness, and as such about being in the world, phenomenology, how we relate to time. Heidegger figures prominently here.