Ben's secret room was an ancient chamber in Ben's house at the Barracks. It was accessed through a false wall in a hidden closet. Its door and walls were covered with hieroglyphs, and the Monster could be alerted by draining a pool of water inside.
The secret closet that provided access to the room could be found behind a bookcase. Inside, a wall panel behind a rack of clothing concealed a much older door covered in hieroglyphs. Beyond this door was a small tunnel that led to a pool of stagnant, brown water. The Monster could be summoned by pulling a plug at the bottom of the pool, allowing the water to escape through a drain.
Several years later, Ben returned to the Island and went to his house to summon the Monster so he could be "judged". He again went into the chamber, and its contents were finally shown. After draining the pool of water, he spoke into the drain, saying that he would be waiting outside. However, only The Man in Black (disguised as Locke) came out of the bushes, not the expected cloud of black smoke. ("Dead Is Dead") However, it became apparent at the beginning of season 6 that the action of draining the pool had indeed functioned as it should, as The Man in Black was indeed the monster. ("LA X, Part 1")
By 2007, Ben had come to believe that the room was actually a manipulation tactic of the Monster. "It’s where I was told I could summon the monster. That’s before I realized that it was the one summoning me." ("What They Died For")
It is highly likely this room was connected somehow to the Tunnels beneath the Barracks.
Please Note - All descriptions of symbols and their meanings are best-guess attempts until official confirmation from the producers of LOST.
There are 8 hieroglyphs carved onto the secret door, although there are only 7 in the Season 4 DVD extra "Mysterious Egyptian Influences".
According to Gardiner's Sign List , the symbols are:
Man with out-stretched arm
Man with weapon
Unclassified, but looks like a Cross
Image from Season 4 DVD.
The first line translates as "summon". According to one rough interpretation, they read, "to summon protection". The second line, as indicated, could mean "protection". This interpretation makes sense in the context of the episode.