It basically means that life has no meaning unto itself, and that we have to give it meaning based on our own ideas/experiences/etc. So, existentialism does deal with some specific topics, but it's more of a method for dealing with problems rather than a set of problems to deal with, if you get what I mean.
What's important in philosophy according to existentialists though, is the subject, their influences (usually their culture), their decisions, their agency, etc. and I think it assumes basic human freedom in a lot of cases, and it really puts a lot of value on the way you think and use your freedom.
An existential hero is someone who goes through the following process:
1. They think about life/morality/etc.
2. They do things in accordance with the values that they have thought about, and in so doing help others gain freedom, or who face danger.
3. They don't assume an identity that they are born with, but rather they work on creating one.
Existentialism really gained momentum after WWI (although, as a school of thought it is much older than that). Philosophers saw the terrible things that happened, the heinous acts of genocide, and they realised that in order to live well, we have to be morally acting people. It's all very well and good to have values, but it's also important to live by and stand up for them. So, an existential hero is someone who does that.