The Registeel Shrine is used to spawn the legendary pokemon Registeel
To spawn Registeel you need 3 Things
1. The Iron Key
2. A Regigigas In your party
3. 9 Unown blocks spelling out R-E-G-I-S-T-E-E-L
Learn how to craft certain Unown blocks Here
Then you need to right click the shrine with the key in hand.