A flagon is a large metal or ceramic pitcher for holding liquor, especially wine. It’s like a giant flask with a handle and spout that you put on the table so you can share the wine with old-timey friends.
A flagon is usually used for alcohol — in particular wine, but sometimes beer. It’s an old fashioned word. A flagon might be set on a table in Colonial times, but feel free to use one now. A flagon full of wine should be enough for a group of friends at dinner — it's more than a bottle. The word flagon is related to flask, which is a smaller container for carrying alcohol.