Solenoids are basically electromagnets: they are made of a big coil of copper wire with an armature (a slug of metal) in the middle. When the coil is energized, the slug is pulled into the center of the coil. This makes the solenoid able to pull (from one end) or push (from the other)
This solenoid in particular is nice and strong, with a 40mm long body and a 'captive' armature with a return spring. This means that when activated with up to 12VDC, the solenoid moves and then the voltage is removed it springs back to the original position, which is quite handy. Many lower cost solenoids are only push type or only pull type and may not have a captive armature (it'll fall out!) or don't have a return spring. This solenoid has 4xM3 (metric) threaded holes in the bottom for easy mounting.