"Existential" relates to existentialism, see
Wiki &
Google Define.
I don't blame you for being confused with the definition of the word. No matter who you are I think it is fairly hard to wrap your head around it. Existentialism is basically a philosophical theory or approach that emphasises the existence of the individual person as a free agent determining their actions though volitional acts. Generally it is atheistic, originating with Nietzsche, but there is a Christian strand that comes from Kierkegaard. Typically it disparages scientific knowledge, denies objectivity, and stresses human experience.