Orebfuscation (a portmanteau of "ore" and "obfuscation") is a mechanic on civ servers managed by the Orebfuscator plugin (source) which makes x-raying more difficult.
Orebfuscator will render several blocks as stone when viewed from a distance. The blocks hidden in this fashion include valuable blocks like ores, and also blocks which generally indicate defensive infrastructure, civilization, farms, bases, or anything else a player might want to keep hidden.
Blocks generally rendered as stone from a distance include snitches (both noteblocks and jukeboxes), obsidian, chests, ender chests, trapped chests, furnaces, crafting tables, and clay.
When a player comes within a certain radius (varies by server, but usually around 8 blocks) of these blocks, they are rendered as their normal block. Even if the player moves outside of the radius, they will keep their normal appearance until the player unloads the chunk.
Even with block hiding, there can be tell-tale signs of civilization or defensive infrastructure, such as long dug-out tunnels or a box of (what appears to be) stone. To guard against this, Orebfuscator also fills the world with fake random valuable blocks underneath the surface, to make it more difficult for x-rayers to see through these blocks.
Blocks that are rendered underground in this fashion typically include every kind of ore, air, cobblestone, mossy cobblestone, stone, obsidian, tnt, and clay.
Unless you're x-raying, you should never see these hidden fake blocks. The one exception is if you mine while the server is lagging - sometimes the server can't send you the true value of a block before you break a block next to it and expose it, resulting in the display of random hidden blocks.
Orebfuscator only modifies what the client sees and does not modify the actual block type of the blocks it obfuscates.