This is a Valley Diverter. It is designed to prevent rainwater from Over Splashing the gutters. See the House below in the upper right corner to see how it will look on Your House.
Valley diverters are generally placed on Inside corners beneath two converging rooflines which make a valley. Because these valleys are draining two rooflines in a concentrated "V" configuration the rain water will tend to overshoot the gutters in a Heavy Stream. The Valley Diverters are designed to catch this stream of water and channel it into the Guttering.