Bucks are one of the two currencies in Legacy of Heroes, and are by far the more common. In the early game, you will often find yourself in need of Bucks to purchase Glory Packs to improve your deck, but in the late game (once you've acquired everything you need from Glory Packs), Bucks are nearly useless, so players can accumulate stocks of millions of Bucks without anything particularly useful to do with them.
Bucks are primarily obtained by winning fights, either in missions, brawls, or PvP. For missions or League Fortress, winning a fight will always give you 10 Bucks per energy used, regardless of difficulty. For brawls, the multiplier is 12 for normal brawls and 18 for crisis brawls. Duels give you 100 Bucks for a win. Bucks are also given out as a daily reward (on day 1). Finally, you can exchange Legacy Coins for Bucks at a rate of 100 Bucks per coin, but this is very strongly not recommended -- there are many, many better things to spend your Legacy Coins on.
The primary use of bucks is for buying Glory Packs. Bucks can also be used to buy Honor Packs (for very new players) and to buy various equipment in the shop, although all of the equipment which can be bought using Bucks has only cosmetic benefits; there's no actual gameplay benefit. There are also occasionally rare cards or rare card packs (such as the Codename Pack) on sale at the Time Traveling Merchant which can be bought with Bucks. However, with the exception of the special, one-time-only Advancement Pack, it's impossible to get any Epic or Legendary card using Bucks; you can only get Rare or lower cards.