What the heck is a Graviton?

Well, nobody is sure they exist. Gravitons are the speculated particle that give objects their gravity. If an object emits additional gravitons, it is essentially equivalent to increasing its mass. Although these particles have not been confirmed to exist, it is one of the most reasonable theories as to why gravity works in the way it does. 

If we don't know they exist, how can we say that's how Mjolnir is so heavy?

Well, we know one thing for certain. If Mjolnir always possessed enough mass that the Hulk could not lift it, it would easily go through the coffee table in this clip:

Since Mjolnir does not go through the coffee table instantly, it can be assumed that it is in some way manipulating gravity in order to prevent those who are unworthy from picking it up.