Before the days of digital reverb, spring reverb was the only cost-effective artificial reverb available. It is still used now in guitar amplifiers.
The spring reverb is an electromechanical device very much like the plate reverb. The signal to be reverberated is fed to a transducer at the end of a long suspended metal coil spring. .The transducer causes the spring to vibrate, and of course it is in the nature of springs to be 'springy', so the signal reflects from the other end of the spring and generally bounces around.
At the other end of the spring is another transducer that converts the motion of the spring back into an electrical signal.
A good spring reverb can give a very good sound on strings, but is hopeless on anything percussive as the springs tend to twang. This twanging is generally unpleasant, although it can be used to emulate an authentic 1960s sound.
The most fun you can have with a spring reverb unit is to give it a kick. The springs (there are often two) will clash together giving you the greatest, wettest thunderclap you have ever heard!