If something is existential, it has to do with human existence. If you wrestle with big questions involving the meaning of life, you may be having an existential crisis.
Existential can also relate to existence in a more concrete way. For instance, the objections of your mother-in-law may pose an existential threat to the continuation of your Friday night card game. Often the word carries at least a nodding reference to the philosophy of existentialism associated with Kierkegaard, Nietzsche, Sartre, and others, which emphasizes the individual as a free agent responsible for his actions.