What are coil-overs/coilovers?

A coilover is an automobile suspension device. "Coilover" is short for "coil spring over shock". It consists of a shock absorber with a coil spring encircling it. The shock absorber and spring are assembled as a unit prior to installation, and are replaced as a unit when the shock absorber has leaked. This provides damping without torsional loads. Some coilovers allow adjustment of ride height and preload, using a simple threaded spring perch similar to a nut. More advanced adjustable coilover systems use a threaded shock body, along with an adjustable lower mount for ride height adjustment, while an adjustment knob is used to adjust damping. Stiffness is changed by switching the spring for one with a different spring rate.

Src: https://en.wikipedia.org/wiki/Coilover