An Iridium Flare is sunlight reflected from a satellite, below is a direct quote from the iPhone app Star Guide.
Iridium 82 is one of 66 active communication satellites launched in the late 1990s and owned by Iridium Communications Inc. The fleet was named after the 77th element on the periodic table—as it was originally intended to consist of 77 satellites—and provides communication services to satellite phones, pagers and other low bandwidth transceivers to any surface location on Earth. The Iridium constellation is configured in 6 polar orbit planes, evenly spaced by 30 degrees with 11 satellites per plane. Each satellite can relay transmissions to adjacent satellites in the network. This design allows for complete coverage of the globe, low latency of transmissions and smaller antennae form factors for hand-held devices (as opposed to geostationary satellites which experience a half-second lag and require larger antennae). Iridium flares occur due to the unique shape of these satellites. Each unit has three rectangular antennae which are silver-coated and happen to be angled in such a way that reflect sunlight back down to Earth. The perceived brightness of each flare is fairly predictable and under ideal conditions can reach magnitudes of −8.5 if an observer is centered in the path of reflected sunlight. Such magnitudes are 1,000 times brighter than the brightest star in the sky, making them a popular target for amateur observers.
An iridium satellite.
Below is a composite of eight(8) images I took from by backyard during the Iridium 82 flare on the 8th of February 2016.