This is a difficult question to answer, because existentialism is such a broad movement that stretches from the 19th -2 0th centuries. Moreover, phosophers who have compeletely different views of the world such as Kiekegaarda nd Nietzsche can be called existential philosophers. With that stated, there seems to be a few common threads in their worldviews.
First, existentialist reject philosophy for philosophy sake, that is, philosophy that does not include the full range of things that make us human, like our individuality, emotions, angst, and the like. Hence, we can say that existentism is a reaction against post-elightenment philosophy.
Second, there is a great emphasis on the self, or the person as individual. This is the key point.
People need to make meaning for themselves. They need to make decisions, exercise free will, take resonpsibility, overcome obstacles and thereby create meaning. As you can imagine these qualities do not necessarily tell you what existential philosophers believe in terms of their worldviews, be in the least we can say that these ideas are central for existentialism.
Existentialists are concerned with how to live one’s life, they believe there are questions such as death, the meaning of human existence, the place of God, the meaning of value, interpersonal relationships, and the place of conscious knowledge of one's self. They believe that life is difficult and that it doesn't have a universally known value. Choices and values should be acted on, not talked about. Free will is an absolutely obvious conclusion of all human decisions.