Thick Skinned is a star within the Constellation The Lady in Elder Scrolls Online.
This was added in Update 6 with the Champion System. It reduces the damage you take from " damage over time" effects by [x]%. You can put 100 Champion Points into this passive ability for a maximum of 25%.
With each point invested, the reduction in the damage taken from " damage over time" effects diminishes. That is to say, for the first 10 CP invested, you would gain a 4.78% reduction in damage taken. However, the next 10 CP spent (making a total of 20 CP spent), will earn you only 4.26% more reduction. This is 0.52% less than the reduction gained in the first 10 CP.
As such, spending just 37 CP into Thick Skinned will already gain you a 15.1% reduction in the damage taken from " damage over time" effects. This means you will have gained 60% of the Maximum reduction possible (which is 25.0%) by spending just 37 out of the total of 100 CP. Therefore, beyond this point, the reduction in damage received per 1 CP used is very small and not as worthwhile.