Havent seen the original in a while.
Do they explain all that in there?
Nah, they don't go into much of that. I'm not even sure how much of that the new movie will cover.
It (character) - Wikipedia
A monster of unknown origins, It originated before the creation of the universe itself in a dimension that is referred to as the "Macroverse"; the true form of It is a mass of destructive orange lights known as the "deadlights". Because of this, the Losers' Club does not know what It actually is, giving the creature its name. It usually takes the form of a clown named Pennywise, which It uses as a decoy when stalking a child. It takes the form of a monstrous black spider in its home under the sewers, where It is revealed to be pregnant.
The monster arrived on Earth in the form of an asteroid during prehistory, landing in what later would become the town of Derry, and hibernated there until humans settled and colonized the area. It awoke, fed on the settlers, and started a cycle of hibernation, in which It would sleep for almost three decades and be awake for approximately two years. Every time its hibernation stage finished, events of extreme violence happened in Derry. To feed on its prey, It transforms into the shape of whatever the victim most fears. Instilling this fear, the creature informs readers, is the equivalent of "salting the meat". It could feed on adults just as easily as children, but it preferred children because their fears were easier to embody into a single shape, making hunting easier.
In the intervening periods between each pair of events, a series of child murders occurred, which were never solved. The book's surface explanation as to why those murders were never reported on the national news was that location mattered to a news story—a series of murders, no matter how gruesome, do not get reported if they happen in a small town. However, the book's implied reason for why the atrocities went unnoticed was far more sinister: It never let them. Since It controls what happens in Derry, many of the child murders are never solved, as the adults of Derry either act as though nothing is happening or have forgotten about the periodic murders and disappearances.
Fred.