The Screw modifier is similar to the Screw tool in the Toolbar, in that it takes a profile object, a mesh or a curve, to create a helix-like shape. Use this tag for questions about the Screw modifier.
The Blender manual entry for Screw Modifier says
The Screw modifier is similar to the Screw tool in the Toolbar, in that it takes a profile object, a mesh or a curve, to create a helix-like shape.
The profile should be properly aligned to the cardinal direction of the object rather than to the screw axis.
The screw modifier is used to generate screw-like geometry from a profile.
Use this tag for questions about the Screw Modifier.