Origin Placement

The function of an object's origin is slightly difficult to explain to someone unfamiliar with the concept, but simply put, an object's origin is its handle. In Blender, an object's origin is represented by a little orange dot.

When you rotate, scale, or move an object in object mode, these functions are performed on account of its origin's location. Blender's mirror modifier is also dependent on the location of the object's origin.

The placement of the origin is something to consider, because in gameplay, it is the point from which the object spawns, as well as the default point from which it is picked up and held.

As a general rule, if the object has a handle, the origin should be placed in the center of the handle. If the object has no obvious handle, then it should go in the bottom center of the mesh.

Another (less common) origin placement would be in the center of a rotating axis, like in the hinges of a door or lid.