Resistance reduces the effect that negative effects and criticals will have on you. For example, if you are hit with a negative effect and have 10% resistance, you will only recieve 90% of the negative effect. Negative effects include Slowing, Jamming, Critical damage and armor and defense erosion.
Resistance reduces not only armor but also defense erosion (degradation) however at half its strength. So 100% Resistance would reduce erosion to 50% of its original value (to be confirmed).
Resistance is a stat which one can include in constructed mechs. Mechs which are bought in the mech shop don't posess resistance and can't be skilled in it. However armor can have resistance. Armor resistance and (constructed) mech resistance should stack (to be confirmed).
Resistance is especially useful to reduce the damage from critical hits.