If you've read the books or watched the miniseries, it's explained in there what IT is, i.e.
An extra-dimensional alien entity
But is this actually explicit in the 2017 movie? IT doesn't leave enough evidence or has enough witnesses, for example:
IT left no wounds like bite marks that can be directly attributed to itself (Eddie breaks his own arm from a fall, Georgie's corpse with bitten-off arm is never found). No adults can see IT; Beverly's father doesn't see the blood, the librarian doesn't notice the lights going off)
This makes other explanations possible, like that it is in the minds of the children, possibly brought on by their traumas in a form of mass hysteria, or that it's some other kind of supernatural phenomenon.
Is it ever made clear what IT is in the movie? And whether that matches what was in the novel?