CP is a measure of the strength of character and team. The following is how to increase CP
Promote
Promote and upgrade hero levels to the maximum to increase CP and attributes such as HP ATK and Armor
Gear
Use at least purple gear so that your CP is high. Gear can be obtained from dungeons or by crafting
Affectio
Give gifts to heroes to increase affection. Affection will give effect such as adding attributes
Skin
Skin provides additional attribute effects. Skins can be unlocked by increasing stars, buying etc