A satellite is visible from the ground when the Sun has set, and it is dark where you are, but the satellite, flying high overhead, is still in bright sunlight. What you are seeing is the sunlight reflecting off of the satellite. The brightness of a satellite depends on several factors:
Like the Moon, a satellite's brightness depends on its position relative to the Sun and the observer. It appears brightest when the Sun is behind you. This same geometry makes the Moon "full" once a month. When the Sun has just set, and you are looking for a satellite in the western portion of the sky, you are looking toward its "dark side". This is similar to the Moon when it is "new". You should generally try to find passes to the East in the evening and the West in the morning.
Maybe the easiest satellite to spot is the International Space Station (ISS), which spans 109 meters (358 feet) and orbits between 370–460 km (200–250 mi) above Earth. Large satellites are generally easier to spot than are small satellites. But size isn’t everything, as the material used to manufacture the satellite can make a big difference. A satellite built with shiny, reflective materials is like a disco ball in orbit, easy to spot, while one with a dark finish might be invisible to the unaided eye.
Even if a satellite is large, it may not be visible if it is not well-oriented to the observer as it passes overhead. Most rocket bodies are large and easy to see. They tend to be cylindrical in shape and several meters in length. If a rocket body is oriented so that it is viewed end-on, like looking through an empty pipe, it will probably not be visible. If the rocket body is oriented so that it is viewed from the side, it will probably be very bright. And if you ever see a satellite flashing intermittently, don’t worry, it’s just tumbling through space, putting on a strobe-light show for your viewing pleasure. I bet it gets dizzy.