okay, I will try to explain what I think you mean, but there are two types of resists.
What I think you are referring to is the heat/cold/matter/spirit/energy/body/crush/thrust/slash resists.
By having higher resists, you take less damage to these types of attacks. Lets say a paladin with a shield/sword is attacking you. Slash resist will help reduce his/her damage. Same for fire damage against a wizards fireball. It is a straight up damage reduction, and the maximum you can get is 26% at level 50 (depending on race, can also have up to 5% more than that).
The other type of resist which exists is spell resist, which basically is just the chance of a spell or effect not working. If you are a caster and you get a message that "the spell is resisted", that just means it did not work. As a caster, if you are getting resisted often, Mastery of Focus can help reduce the resist rate.