In game, you might notice some sections of the floor with runes marking them. These sections are breakable and allow you to access new areas. 

Before you are able to unlock the ability to break floor Runes, you must first have the ability to use the teleportation statues. These are the purple statues you can rub. Once you have this ability, progress through to the Promenade of the Condemned biome. Once you get there you will need to keep exploring until you find the exit to the Ossuary.

The location is random but it is always behind an area that requires a teleportation statue to reach. Once you have located the exit, simply head through. This can be a very large area so it may take some time. You need to explore until you find an Elite enemy. Specifically, BLANK. You may come across the entrance to the Black Bridge but avoid taking that until you have completed your goal.

Explore the entirety of the Ossuary, until you find an Elite room. You will have to battle an elite skeleton enemy here. Defeat it for the Rune that unlocks the ability to break through floor tiles to unlock new secrets.

In order to break them, you need to do a power jump. To do so, position your movement joystick downwards to make your character crouch and simply tap twice rapidly on the "Jump" button.