A flow-map is a texture that stores 2d directional information in a texture. The color of the pixel determines what direction it is using the uv-coordinates texture as a base. The more color there is the faster the proportional speed. Example green tells it to go up-left, the center is neutral, and red will go down right. It’s a useful technique for liquid materials like water, and a useful alternative to just a panner node.
A tool for making a flow-map: http://www.polycount.com/forum/showthread.php?t=105399