What is it and what is it’s function in a car?
It is actually spelled solenoid, and is basically a rod that slide through a winding that is given power. As it gets power it creates a magnetic field which moves the rod back and forth. They are basically used as actuators to move or switch something else.
For example, starter solenoids are part of a car’s starter, and move the drive gear in and out of the starter’s housing, usually by moving a fork, to move the gear against the teeth of the flywheel. The starter can then rotate the flywheel to begin the engine’s end of the starting process.