Entities are placed in the world. The camera moves on a path, and most of the time, when the camera “sees” an object (like an enemy) it wakes up and starts to move. This behavior is the default behavior for every object in the game.
But if you check the 
Attached to Camera checkbox on enemies or items, this is what happens: the camera moves on the path, see the objects, and when the object wakes up, it got attached to the camera.
Think of a wire attaching the object to the camera: when the camera moves, the object moves along, and what you see on screen is a static object.
The object can move like before, but it will move along with the camera on top of its own movement.
Note: one thing to remember is that an enemy will wake-up when the camera Wake zone touches it. So, most of the time the enemy will attach itself to the camera when it's not or almost not visible. This is why you should add an entering movement with a path. (Gif)
Note: the player is always attached to the camera. Its position if an offset from the camera, not a world position.