The character is an ancient, trans-dimensional evil entity which preys upon the children of Derry, Maine, roughly every 27 years, using a variety of powers that include the ability to shapeshift, manipulate reality, and go unnoticed by adults.
See also: It (miniseries), Pennywise, Pennywise (band), It (2017 film), Georgie Denbrough, Pennywise (album),