The only existentialist with whom I am familiar is Sartre, so I may be influenced by his particular viewpoint. I also recommend reading Sartre’s Being And Nothingness, but be prepared to skim over sections, because the book is in bad need of editing.
Existentialists believe that we know that we are but not what we are. They believe that there is no inherent meaning in life. We are forced to define ourselves and to create our own meanings.
Some view existentialism as bleak and others as liberating.
When I was younger, I was greatly influenced by Sartre. My feeling now is that much of what he says is brilliant, but he focuses too much on the individual. Sartre considers others as a threat to an individual’s integrity. This makes for a rather lonely life. Surely some of the best times most of us have is in communicating with and sharing experiences with others.