Armor helps you in battles by taking in part of the dammage done to you.

Although, The dammage taken is by the armor is dealt to the armor as well.
Example:namek armor takes in 25 % of a kamehameha that deals 1000 dammage so it takes in 250 dammage so the armor is at 49 750/50 000.

Once any armor reaches 0/out of it's limit , it is destroyed. You may repair your armor at the capsule corporation or if you have blacksmithing proficiencies.