You, an ironsmith, create a horseshoe. You are the cause of the horseshoe's coming into existence.
However, the iron's solidness, which is based upon further factors such as the strong and weak nuclear forces, the state of electrons in the iron, and so on are the causes of the horseshoe's continuing existence. These causes sustain it in existence from moment to moment.
The first type of cause is called "in fieri." The second type is called "in esse."