A solenoid is a coil of wire that generates a magnetic field when an electric current passes through it. This magnetic field can be used to create motion or control devices, making solenoids essential in various applications, such as in electromagnets and relays. When the current flows through the solenoid, it transforms electrical energy into mechanical energy, allowing it to move a metal core or plunger. This principle is utilized in many everyday items, including door locks and automotive starters, demonstrating the solenoid's importance in modern technology.