Using a braille decoder.
The braille in Registeel's tomb on Poké Emerald says: "those who inherit our will, shine in the middle", which means use the HM move flash in the center of the the tomb.
A braille chamber is a puzzle which you must solve in order to unlock certain Pokemon, such as Regice, Regirock or Registeel.
to catch the Regis, you first have to complete the underwater cavern somewhere around sootopolis. it REALLY helps if you can interpret braille. after that, to get to registeel's cave, which is south of fortree, on a side path, then follow the braille instructions in that cave. then a passage should open to registeel.
The three regis, regirock regice and registeel.
The Braille in the Ancient Tomb translates to 'Those who inherit our will, shine in the middle.' . This means you have to walk to the center of the room and use the HM move Flash. This should open the inner chamber containing Registeel.
Walk to the middle of the cave and use flash.
You go inside his cave, and walk straight ahead and it's on the wall there.
You read the middle braille, head for the middle of the room, and use fly. the door should open, and you'll be able to fight a level 40 Registeel.
You will see the chamber and a wall with braille on it. The braille says 'Dig Here' You will soon gain access into the first sealed chamber. The wall will open (eventually) allowing you to catch Regice.
In the Sealed Chamber, there are multiple braille clues for you to uncover. These clues tell you how to get into the Chambers of Regirock, Registeel, and Regice, respectively.
The Braille Decoder sheet for unlocking the Regis in FireRed and LeafGreen was sold in the official guide released by Prima for the games. The Braille used in the games as a visual representation of the raised bumps and spaces of the braille language.