bjwitham: First, you should realize that Slender and Slenderman are simply internet names that came out of the creepypasta revolving around the origin of the character.
Here's the thing: we shouldn't need to. If the game truly wants to stand well on its own, it needs to weave stuff that the player is expected to know into the exposition. That said, I greatly enjoyed the original Slender without knowing anything about the mythos of Slenderman; all I knew was that I was in the woods and that a scary-looking thing that looked human except for its odd proportions, blank face, and seemingly unnatural speed which he ceased taking advantage of once I looked at him (which on its own gave me uncomfortable flashbacks to the Weeping Angels from Doctor Who), and that I knew nothing about him but that I should consider his intent hostile, resulting in me running for my life and being genuinely freaked out. The fact that so much was unknown about my pursuer made it even more horrifying, almost akin to Lovecraft's tendency to barely describe the creatures that appeared in his stories both in terms of their appearance and motives (especially their motives), except while a Lovecraftian abomination would echo deep sea creature blended with gigantic space monster, Slender made its antagonist creepier by having it strongly resemble something one would see in everyday life, a man in a suit, but with little things like the creature's too long arms and legs, its blank face, and the way it always seemed to get closer only to stand still when gazed upon, almost as if it were trying to psychologically attack me, combined with the pants-shitting realization that I had no idea what the hell the thing was going to do to me. It took the unknown, something that is becoming a rarer commodity in this day and age, and weaponized it. Knowing the exact details of the Slenderman mythos was interesting, but if anything, it diminished the experience, because it now gave me a name to assign to my enemy and an idea of what it was capable of, diminishing the mystery that made the original game so scary.
As to your post indicating that people need to know this in order to enjoy the game, I surmise that this was posted in the belief that people do not like this game have not immersed themselves in the mythos, I would like to draw your attention to the fact that the chief complaint voiced by almost everyone in this forum is not how they don't understand what is going on, but that the game was apparently designed by a retarded chimpanzee in that it is so poorly optimized that even fairly powerful computers are not able to run it at a satisfactory frame rate. I picked up a top of the line Thinkpad 2 years ago which has run some majorly demanding games no problem, but this game consistently runs like an early 20th century film with every other frame missing and played in slow motion; somehow an indie game has a poorer framerate than any AAA title I have played on this thing,, and as far as I can tell, there is no reason for this, as I have played games from roughly 10 years ago with better graphics.
tl;dr: why defend the story when the problem is that the game seems to be designed to run on the Monolith from 2001 despite looking like a game from last decade?