The rules will affect ATM withdrawals and debit card transactions, and are meant to protect consumers with an opt-in option. What does this mean?
Previously, banks would allow you to overdraft your account, and then because you over-drafted, they would hit you with their large fees. My bank was $29 per overdraft last I checked.
Now, they are not allowed to let you overdraft unless you explicitly ask. I think the idea is that since most folks won't purposely overdraft very often, they would rather be declined the purchase than pay a $29 fee.
Imagine if you overdraft your account buying $75 worth of groceries. The fee just made your bill $100 and you probably didn't know. However if you were declined, then you would have the option to just take things out of your cart, buy the $40 in groceries you can afford.
The banks hold the position that if you are lost on a dark lonely road and the tow truck guy will take your debit card, if you can't overdraft you can't get a tow home. They call it a favor to you. I disagree.