Solenoids are basically electromagnets: they are made of a big coil of copper wire with an armature in the middle.
When the coil is energized, the slug is pulled into the center of the coil.
This solenoid in particular is nice and strong, and has a slug with a slanted cut and a good mounting bracket.
Normally the lock is active so you can't open the door because the solenoid slug is in the way.
The solenoids come with the slanted slug as shown above, but you can open it with the two Phillips-head screws and turn it around so its rotated 90, 180 or 270 degrees so that it matches the door you want to use it with.
To drive a solenoid you will a power transistor and a diode, check this diagram for how to wire it to an Arduino or other microcontroller.
You will need a fairly good power supply to drive a solenoid, as a lot of current will rush into the solenoid to charge up the electro-magnet, about 500mA, so don't try to power it with a 9V battery!