OK basically existentialism fundementally means that we all have a certain amount of freedom to direct our own lives. Why choose one action over the other?
There is an inifinite amount of consequences each action can give and divert you to.
Many existensialists questions where this free will comes from, some say its connected to God (Thiestism) who say we need to be alligned with a greater design plan to our own, others are Athiestic (Nihilism) who usually say there is no other meaning to living other then breathing, excreting and reproducing and others Agnostic (Absurdism).