By default luck increases the chance of seeing rarer cards. but it can also be used by other mods to improve the likelihood of chance based effects.
to have a card adjust a player's luck, add the Luck component to a card either in unity or in SetupCard, then set LuckAdd and/or LuckMult accordingly.
to have a luck based chance event, call Player.GetComponent<Luck>().RollLuck(chance) where chance is a float between 0.0 and 1.0 (inclusive) representing the base % likelihood of the event occurring.
if you are using the CustomCard component to make your card, in SetupCard you want the following code:
Luck luck = cardInfo.GetOrAddComponent<Luck>();
luck.LuckAdd = //the value you want to increase the player's luck by
luck.LuckMult = //the value you want to multiply the player's luck by
// note you likely wont be using both Add and Mult in the same card.