Existentialism's bumper-sticker: existence precedes essence. You're not born with a fixed purpose (the way a knife is born for cutting). You exist first — a raw conscious being thrown into the world — and only later do your choices make you into who you are.
This is freedom, but it's also a weight. Sartre called it "condemned to be free." Without a god-given meaning, every important question (what to live for, what's worth dying for, what kind of person to be) is on you. No appeal to a higher authority can answer it for you, and the temptation to pretend otherwise — to live as if some authority has settled it — is what Sartre called bad faith.
Existentialists don't agree on much else. Kierkegaard was a Christian; Sartre was an atheist; Camus rejected the label. What they share is a refusal to look away from the fundamental strangeness of being a self-aware creature who has to keep choosing, with finite time, in the dark.