The Starfire crystal is energy in solid form composed of an undetermined number of intelligent beings from another dimension. When the Starfire entities decided to explore our universe, they were forced into the crystalline matrix to maintain their integrity. Just as matter cannot exist in their dimension their energy would be dispersed in our universe. They came as explorers to learn about the creatures inhabiting this universe because they realized that life created energy which passed into their dimension. They record all that they experience in perfect clarity but are limited to travel by the laws of our universe and the creatures that may bear them.
The Starfire portal is a hole through the dimensions of our universe and the dimension of the Starfire entities. Because the Starfire entities' dimension overlaps our universe, energy from one to the other can transfer anywhere instantly, otherwise matter would disintegrate. Matter cannot exist within the dimension of the Starfire entities.
The Starburst marks are an eruption of the Starfire's presence within the body of a Keeper. The greater the presence of the Starfire energy within the cells of a Keeper, the larger the marks will appear. Also, the longer an individual is in contact with any part of an actual crystal, the more energy they absorb and the larger the marks may grow. The latter is true of Saffir, whose entire hands and forearms are encompassed by the marks.
Starburst marks are what make Keepers unique. They are a conduit through which the energy of the Starfire flows (usually out but they can also be used to absorb energy as directly from another Keeper). They usually appear on both the palms and backs of the hands but in partial Keepers are only present as a very small mark on the palms. Only with the presence of the Starburst marks can an individual expel any of the energy charged from the resonance within the individual.