Resistance Bonus is a bonus featured on some items, the amount indicated is added to your total Resistance.
With no Bonus, your base Resistance is equal to .5% per 50 points in your "spell resistance" skill. this means that, with no extra bonuses, you can have a max spell resist of 10%.
Unlike protection, spell resistance does not mitigate a percentage of incoming damage, but rather has a percent chance to ignore each damage die from any magic atack or spell an enemy uses againt you.
This means that if you had a 10% magic resistance you would have a 10% chance to ignore each damage die from any magic attack or enemy spell used against you.
It is important to point out that this makes resistance one of the best skills to get in the game, due to the highly damaging nature of enemy spells and the tendency to have a lower magic defence than your weapon defence.