The stiffness, or modulus of elasticity, describes tensile elasticity or the tendency of an object to deform elasticly as opposing forces are applied to it. It is often referred to simply as the elastic modulus or E-modulus. The most common SI Unit for the E-modulus is Pascal (Pa) and because the value is usually large is used Giga or Mega Pascals (GPa, MPa).
Unreinforced plastic have a low E-modulus and thus deform easily when subjected to a load. Steel have a high E-modulus and is very rigid. To compare the two you also need to take into consideration their specific stiffness.
In many application a relatively high E-modulus is targeted to avoid deflection under load. This e.g. due to risk of dynamic movement, swaying, which can cause resonance problems and more.