A variety is resistant if the pest can not eat it, or can not live or multiply on it. Thus, if a plant is resistant against a pest, it will not suffer any damage from that pest.
For example, certain cotton varieties are resistant against jassids because the plants have hairy leaves. The hairs form a barrier for these small insects.
Note that a resistant variety in not the same as a tolerant variety. Compare tolerance and resistance in the picture below.