Here is an excerpt from another forum. Basically, in the case of an aircraft with an engine type such as the PT-6, as found on Twin Otters, King Airs, etc, it's a fuel flow control lever:
Condition levers control fuel flow. The condition levers in the King Air have 3 positions, Cut-off, Low-Idle, and High-Idle. By moving the condition levers to Low-Idle, you set the engine to idle at a specific speed, moving them to High-Idle allows the engine to idle at a higher speed by introducing a greater constant fuel flow. Low-idle is used for starting and taxi; High-idle is used for normal flight and any time that that extra bleed air is required (such as for the air conditioning with only one engine running). Cut-off is just that, it cuts-off fuel to the engine.
In some other aircraft, such as the ATR series, the condition lever control both fuel flow and prop pitch.