A thrower is a helper virtual object that facilitates the mapping of content onto various viewport objects by taking into account their position in the 3D scene of the viewport.
There are 3 types of throwers, each one of them projects (or throws) content in a different way:
A Perspective thrower throws the content similarly to how a projector would.
Cubic throwers are used to throw 360 content that uses cube mapping. It is commonly used for projcting textures from Unreal Engine.
Spherical thrower is used for projecting 360° videos on 3D objects. It is used with equirectangular 360° videos.