Amazing things happen whenever the octagonal key is placed inside the keyhole. This activates the cave and allows Jor-El to communicate with the outside world.
The first time Clark did this was in "Rosetta." The red, yellow, and blue hint are hints of the future Superman costume's colors.
As you can see, the key fit into the keyhole so the SM and "hope" symbols on the key matched up with the matching symbols on the keyhole.
A wider shot of the keyhole after Clark put the key into it:
Finally, this is what the keyhole looked like right before it blasted Clark with a beam of energy that contained knowledge of the Kryptonian language.
Note that the hole in the key that appeared and was the source of the knowledge beam is shaped like the famous S-shield.
After Clark left town and headed to Metropolis to escape his destiny ("Exodus), Jonathan went to the caves to angrily yell at Jor-El ("Exile") He flung the octagonal key on the ground in disgust, and the disc glowed, began to levitate, and propelled itself into the keyhole. As before, the mechanism was activated:
