They each orbit about 470 miles high and make about 14 trips around the Earth per day. There are thousands of satellites of all types orbiting the Earth constantly. Some of these can be seen on any clear night right after dark. At that time, the sky is dark while the sun is still shining at the satellite's location. Satellites are then seen shining in the sunlight as a moving point of light against the stars.
Iridium satellites can flare suddenly in brightness.
They each have three highly reflective mirror-like antennas each about the size of a typical home door. These antennae are in a constant orientation with the ground. Because of this orientation and the known orbits of these satellites, it's possible to predict when and where a brilliant reflection of sunlight can be seen by observers on the ground. This reflection can be very bright for as long as 20 seconds when the antenna perfectly reflects the sunlight. The flares can exceed the brightness of Venus and some have even been seen in daylight.