The desire for human connection, when the reality of everyday life is reduced to a cacophony of physical and verbal abuse. The tragedy when your parents are absurd clowns, who rob their son his right to a decent life. The grandmother is a symbol for affection and warmth, and Lynch mixes that with ghostly faces and pitch black rooms. Ultimately, getting love and then losing that makes life a very surreal experience.
You can watch it here http://bit.ly/aE47eC. Though this has a third of Eraserhead's length and kind of the same feel/ideas, Grandmother has the same amount of story in it and it is way more creepier. Lynch's fisrst little masterpiece.