Iridium flares are extremely bright flares observable on the sky, caused by the reflection of sunlight on passing Iridium satellites. The flares usually last for a few seconds and are an interesting phenomen to observe on the night sky. Intensity of flares vary, with the most intense being visible also at day time.
Occurance of Iridium flares is predictable. You can check the occurances in your neighborhood on the Heavens Above website.