The invisible flat reference surface Revit uses whenever you sketch or place something - by default it is tied to the current view or level, but it can be set explicitly to a face, a reference plane, or a specific grid to sketch somewhere other than flat and horizontal, like on a sloped roof or an angled wall.

Confusion about "why did that element go where I didn't expect" is very often a Work Plane problem in disguise - Revit placed the new geometry relative to whatever plane was active, not necessarily the one the user assumed. Understanding work planes explicitly is one of the fastest ways to stop fighting Revit's placement logic and start using it deliberately.