Slartibartifast is a person that you meet later on in the book when the Heart of Gold touches down onto the planet of Magrathea and everyone but Arthur goes through the portal that takes them to Deep thought the massive robot that lives on Magrathea, this is when Slartibartifast comes up to Arthur and takes him through the new earth that has been created for Arthur with all new people on it as well.Then as a final surprise Slartibartifast takes Arthur to his newly made house as perfect as it could be.
Slartibartifast is a symbol because Slartibartfast is a custom planet designer. More specifically, he designs narrow coastal inlets and was actually the person who designed the coastlines of Norway. When Arthur encounters him after landing on Magrathea, he explains to him that he along with everybody else on the planet have been in hibernation for five million years, ever since the galaxy went into an economic recession.
Slartibartifast is very secret of his name because when he went to talk to Arthur and Arthur asks what is your name he just says "that is unimportant so just come with me". But when Arthur say I'm not coming until you tell me who you even are he gives in and says "Slartibartifast" Arthur replies with "What" and Slartibartifast says that is why it was unimportant. I'm sure you think it is a very strange name as well.