It can be used at the Lihzahrd Altar (as you obviously know) located at the end of the Lihzahrd Dungeon to summon the Golem boss.
The Lihzahrd Dungeon can be found under the Underground Jungle, but for access to it, it requires a key that drops from Plantera - which is a hardmode boss that can be summoned by destroying a pink plant in the Underground Jungle.
Note that the plant only spawns after another hardmode boss has been defeated.
The problem was... my Terraria Map seed didn't spawn an Altar. LOL
Anyways, been playing on JC's server.