PHRASE: The tradition of all the dead generations...

"The tradition of all the dead generations weighs like a nightmare on the brain of the living."

I think Marx is saying that no matter how hard we try to come up with something that is new and not repetitive the more likely we are to draw on things from the past.  There is no way that things that have happened in the past and in our own personal pasts don't creep up into our thought processes now.  We are always creating a copy of a copy of a copy and so on.  It's seemingly impossible to come up with anything completely new and not a reflection of anything else.