If you spawn in limbo above this and fall on it, you receive appropriate fall damage and it will not teleport the player, so you have to move to another eternal fabric block for it to work.
Eternal Fabric is a block from Dimensional Doors mod. Found naturally in the lower levels in Limbo, and 4 blocks under the Fabric of Reality in Pocket Dimensions. It is unbreakable by any tool, however it can be blown up with TNT. When stood on (in Limbo, in other dimension nothing happens), it teleports you to the overworld at a seemingly random location with all of your items intact. This is one of the few ways to exit Limbo safely. Another way is to find a "pool" of Ancient Fabric.
You can see through the block, but will not see items and blocks.