Check valves are put in place to prevent backflow through piping. A swing check valve is a straight pipe device that consists of a flap held closed by a spring. The flap is opened by fluid pressure. Should pressure be put on the spring side of the flap, it shuts, preventing backflow.
An angle check valve serves the same purpose as a swing check valve, differing mostly in shape. Angle check valves are L-shaped rather than a straight. A swing check valve should be used with straight line piping, whereas an angle check valve should be used when flow needs to change direction at the check valve. You can use an angle check valve in areas where you might use a piping elbow.