This doesn't target the player or permanent it will enchant, so an opponent's permanent with hexproof may be chosen this way
Urza, Planeswalker Meld Meld is overpowered, but the Meld requirement makes enough sense of both design and flavor
The card then tells you what happens if the kicker cost is paid
You get to tap your rocks for mana, bounce them back to hand, and then recast them to increase your storm count and hopefully even net extra mana, although that requires more artifacts that produce more mana than they cost like Sol Ring, Mana Vault, and Grim Monolith
Also, since the Burning Abyss monsters have their effects activate in the GY, youll have the advantage with cards whose effects activate in the GY when you send monsters you control from the field and hand to the GY