D-bills are a unit of the currency used in Cappy Town and Dream Land in the anime Kirby: Right Back at Ya!. It is presumably of King Dedede's manufact since the note features his face and symbol on it.
D-bills is very similar to modern currencies, with printed notes as well as coins. The coins look like regular large gold coins (although sometimes silver coins are seen), the obverse being the relief of King Dedede's face in the like of his many statues, and the reverse being his peace sign. Likewise, the notes have both his picture and his peace insignia.
Most of the time, the values on the notes are never shown in enough detail to see the values, but in the episode Don't Bank on It, stacks of 1,000 D-bills are seen being used by brainwashed Cappies to purchase stickers and books.
Outside of that episode, no Cappies are ever seen performing particularly large transactions, but King Dedede himself has a huge safe in his castle filled with D-bills, besides a crate of jewelry about as large as himselfwhich represents much of his fortune.
Interestingly, Nightmare Enterprises is willing to accept D-bills as tender although King Dedede's influence does not extend outside of Planet Popstar. Urameshiya was seen transporting notes from King Dedede's safe into the teleporter to cover his debt, so it suggests D-bills have value in one way or another elsewhere in the galaxy.
As a Plot Device
Several of the episodes center around King Dedede trying to get more of it, either to just hoard them or use them to pay off his mountainous debt to Nightmare Enterprises.
While usually a miser, Escargoon liberally hands out individual notes, handfuls, or even whole stacks of them to bribe Cappies to let them have their way with things. Judging from the Cappies' reactions, even a single D-bill can be worth quite a bit, which suggests that different denominations are printed since stacks of 1,000 D-bills are used to buy just stickers. Certain decorative jewelry aside, the "Gold" currency from The Great Cave Offensive in Kirby Super Star and Kirby Super Star Ultra is never given any mention in the anime whatsoever.
NME Salesman always takes issue with the billions King Dedede always seems to owe, and voices his dissatisfaction or contempt by showing Dedede's newly-incurred or standing debt on his calculator, which always shows the sign "D" for D-bills.