They reflect sunlight back out the window before it can turn into heat. They block the movement of hot air from the area around the window into the room. They insulate the room from the hot surfaces of the window glass and frame. Your shades do not need all these features, but the more they have the better they will work. Roller shades work best, followed by venetian blinds.
Curtains help only a little.
In the summer, the sun beats down on your roof for many hours each day. This is one of the biggest sources of excess heat in your home. On sunny summer days, the air in your attic may get very hot. Heat from the attic moves through the ceiling into your home. If your home does not have an attic, the heat travels directly from the roof into the living spaces.