https://m.youtube.com/watch?v=ZdmPIpQZPRg
Effectively it’s pickled pork and can last for a few months. To eat it, you soak it to get as much of the salt out as you can. Good for stews, gravy, etc.
It’s what was done pre-refrigeration for meat.
The meat was edible straight from the jar, the fat was used for cooking or as spread (but tasted quite disgusting to me heh).
I reckon salted it would last even longer.
The meat is often salted a little before cooking. Stored in cellar/refrigerator, it keeps a long time.
You can also buy canned confit duck. Can be eaten straight from the jar, buy delicious and crispy when (re)cooked.
The lard or (even better) duck fat used to preserve the meat can indeed be used like butter. But, by far, the best use of the fat is roast potatoes.
Parboil peeled (preferably floury) potatoes. Whole or cut works. Àdd fat. Jiggle around in a pot to rough up the outside of the spuds. There is no such thing as too much fat, use it all. Roast until crispy and golden brown... 30 to 90 minutes depending on heat and specifics of the potato.
Serve with butter, gravy, onions, sour yogurt, etc.
If you're curious to learn more (and you should be!) I recommend "Charcuterie: The Craft of Salting, Smoking, and Curing", by Ruhlman and Polcyn.
I have about worn out that book. Even has fabulous non-meat recipes. I use the smoked ham recipe in the book to convert industrial $2/lb bone-in pork butts into a glorious thing.