The Regirock Shrine is used to spawn the legendary pokemon Regirock
To spawn Regirock you need 3 Things
1. The Rock Peak Key
2. A Regigigas In your party
3. 8 Unown blocks spelling out R-E-G-I-R-O-C-K
Learn how to craft certain Unown blocks Here
You then need to find a Regirock Shrine. These spawn in Dry biomes or Mesas
Once you have all items required; and you have found the shrine you need to place all 8 Unown Blocks onto the Temple Pillar blocks, (in the right order, spelling out Regirock with them.
Then you need to right click the shrine with the key in hand.