3D Cursor
The marker that decides where things appear
Also known as: Blender 3D cursor · Blender cursor reset · Blender Shift C · Blender snap cursor · Blender cursor to selected
Definition
The 3D Cursor is the red and white marker in Blender's viewport. New objects appear where it is, it can be used as the pivot for rotation and scaling, and Set Origin can place an object's origin on it. Shift+right-click places it, Shift+S opens the snap menu to move it to a selection or to the world origin, and Shift+C resets it and frames everything. It is one of the most useful and most confusing tools for beginners.
In production
If new objects appear in odd places, reset the cursor with Shift+C or Shift+S > Cursor to World Origin. Use Cursor to Selected before adding an object where you want it, and set the transform pivot to 3D Cursor to rotate around a custom point. Keeping the cursor at the world origin during modeling avoids offset problems when you use mirror modifiers and export.