Pond, an FFL stands for a Federal Firearms License (or Licensee). You have to have an FFL t deal in the business of buying guns or manufacturing with the intent to sell. There are different types of FFL's. C&R holders are one of them. It means that you have a Federal Firearms License to trade Curio & Relic firearms. FFL's can ship across state lines to other FFl's. In this way, C&R holders can ship C&R guns to each other across state lines without a background check, because they've passed the checks to get the license and have requirements about keeping records.
You have to have an FFL if you make a business of selling guns. You can collect them and sell as part of a collection, but when you start dealing in large volumes and making money enough that it's part of a business, you must get an FFL.
If I want to sell a handgun or long gun, in most states, I can make a private sale to someone of age in my state without going through an FFL or background check. However, if I list a gun for sale on Gunbroker and somebody from Texas (out of state) wants to buy it, I must ship it from my FFL to theirs. In this way I can sell the firearm across state lines if it is legal to own it in the state I am sending it to. The buyer must then pick it up there and pay for the shipping, FFL transfer, and $5 for the background check.
It's not just about selling across state lines, but that is the way that most people tend to deal with them because that's all many people use an FFL for.
If I wanted to own a gun shop or become a manufacturer of guns for sale, I would have to obtain an FFL and do all that comes with that. Another interesting thing is that FFL's, with demo letters, may own automatic firearms that the general public may not own for the purpose of demonstrating them to law enforcement and (maybe?) military.
FFLs are bound by rules that private sellers don't have. For instance, I'm 18. In my state I can buy a handgun from a private seller or receive it as a gift, but to buy it from an FFL I must be 18. Same with handgun ammo and lower receivers for firearms. I can also buy a long gun from an FFL in a different state, but not a handgun. Lots of peculiarities of US federal and state laws.