Camber refers to the curved shape of a snowboard. When you're looking at snowboard specification, you'll find that hybrid camber comes under many different names. It's labelled differently by different companies, mainly because they are trying to differentiate their variation on this deign by giving it their own name.
Hybrid camber includes elements of both rocker and traditional camber. With rocker or traditional camber, a board will curve in only one direction. A board with hybrid camber will curve in different directions in different sections, resulting in a kinked curve. Think of an 'M' or a 'W' shape.
This type of camber is an attempt to mesh the advantages of a traditional camber board (stability at high speed and improved carving and 'pop') with those of rocker boards (float in powder, ease of turning, easier butter and presses). The result is usually a board that's good for everything, but excels at nothing.
Camber is the curve of a snowboard, so, as one would expect, a board with flat camber (also known as zero camber) has no curve. So, if you place the snowboard on a flat surface, the entire bottom of the board (except the tip and tail) will sit flush with that surface.
Flat camber is a middle ground between traditional camber and rocker and is often used in terrain park boards. The lack of curve makes them stable on rails. They are also better for on-ground tricks, such as presses and butters, than a board with traditional camber.