Typically, an Image_Map node would be connected to Displacement and the displacement map could look like a bump map image; a grey scale image where the values in the image determine the direction and degree of displacement.

You have to make sure you turn on 'Use Displacement Map' under Render Settings when using the Firefly Engine.

Technically speaking, the displacement map doesn't have to be grey scale and you can plug other nodes into Displacement to achieve different effects but an Image_Map node with a grey scale map is usually how most people use it I believe.