Features you can see on seismic that are caused by the seismic wave passing through a gas or oil filled section rather than a water filled section. These show the potential for gas/oil-water interface within a structure. Flat spots can be flat or tilted depending on underlying pressure gradients and structure. They can represent (if they are real) the interface between gas and water, gas and oil, oil and water. Invariably in a typical reservoir you have gas on top (lightest), oil in the middle and water at the bottom.