Crones are the standard currency used in Rohan: Blood Feud. Crones are dropped by almost every creature in Rohan, and you can use them to buy and sell equipment, potions, other items, or to found a guild - and more.
It's common to add a k behind large amounts of crones. This most likely derives from the latin kilo-, which means "thousand". For example, 1k means 1,000 crones, 10k means 10,000 crones and so on.
It is possible to deposit any amount of crones you are carrying in your treasury. Treasuries are located in every town, and should be easy to locate using the map (M key).