Basically, ads are referring to donation. When you donate money to RO using Paypal (and you use Paypal no matter which payment method you use) the money is credited to your forum account (yes your forum account has money associated with it). You can exchange this money for various helpful in-game items in the item shop.
The reason for calling them ads is because you can get them other ways then just donating, which is why they are talked about so much. If you
vote for our server (you can do it every 24 hours for most sites, 12 hours for some - just make sure you don't try to do it in less time than specified, as that is illegal!) you get a small amount of ad credits per day. It's not much, but you can get 1 ad ($1 credit) in two or three days. Do this enough, and eventually you'll have enough to purchase a good card or item from the item shop.
Ads can also be transferred to other players. Since ads are important to the economy, people often want to buy them. An ad can be sold for 850k-950k zeny, which is a pretty good amount of money for a newbie on this server. This is done via the transfer button
on this page: when selling ads, just look for someone who is buying. You can get the zeny from them first or send them the ads first (they will tell you the account name, id number, and amount to send) - if you send the ads first, make sure you take screenshots of them promising to pay you zeny, in case they try to scam you. Don't try to scam anyone else, by the way, as you'll get banned for doing it. If you have lots of zeny in game, you can buy ads from other people this way to get items - it just depends on what you have (ads or zeny) and what you need.
That's about all I can think of, if you have any more questions ask away (or PM me).
