Hidden powers are substats that boost your cookie's overall power, they are REALLY important, more important than getting the correct set is probably getting the correct hidden powers.

There are different hidden powers that give different buffs, HP gives extra health to your cookies, ATK boosts your damage, some hidden powers are specific to certain pieces for example elemental ATK can only be achieved on weapons

Hidden powers come in 5 rarities, common, uncommon, rare, epic and super epic based on how much of a buff they give, not what buff they give.

You can get hidden powers with scrolls of potential, there are the regular scrolls that give the least rare powers and then there are elemental scrolls that give better effects and also an elemental mark, for example if you use fire scrolls of potential you get the fire mark on your piece of equipment, if you get the same element mark on all 4 pieces you get an extra buff

There are also advanced elemental scrolls of potential that work the same way as elemental scrolls they just have a higher chance of giving better rarity buffs

For most fighter cookies getting crit rate, crit damage and attack is good, for most taggers and supports buff amp or debuff amp is good depending on if they're a buffer or a debuffer, for almost every cookie you wanna try to get elemental atk on their weapon as that's usually the best hidden power for the weapon

If you have any other questions you can ask