I am not sure where you are looking, but ToSBase has some good descriptions of the attributes http://www.tosbase.com/database/attributes/
All of the attributes with multiple levels will say how they get better with multiple levels invested.
…except the Cloth/Leather/Plate masteries. Those it doesn’t provide any info on.
Reddit to the rescue! 
Cloth Mastery, 3-piece bonus: +10 SP per Level
Cloth Mastery, 4-piece bonus: +0.5 Magic Defense per Level
Leather Mastery, 3-piece bonus: +1 Evasion per Level
Leather Mastery, 4-piece bonus: +0.5 Evasion per level
Plate Mastery, 3-piece bonus: +34 HP per Level
Plate Mastery, 4-piece bonus: +0.2 Stamina per Level
Disclaimer: this was posted during CBT2, so possibly could have changed
Src: https://www.reddit.com/r/treeofsavior/comments/3qt5z1/cloth_leather_and_plate_masteries/