Because bacon makes everything better, it just does, this is kind of a cheap trick. Nonetheless, it's a tasty cheap trick, and it's a quick cheap trick, and few things are better than quick and tasty cheap tricks, am I right?
Braising lentils just means cooking them gently in liquid (water, stock, unicorn tears), in this case with some onion, garlic, bacon, and rosemary. You can use approximate quantities. For a cup of lentils about two slices of fatty bacon or pancetta works well. Chop it up into a small dice and fry until most the fat is rendered. Cook a chopped onion and a clove or two of garlic in the bacon fat, and when the onion's soft and starting to brown throw in the rinsed lentils, a few sprigs of fresh rosemary, some salt and pepper, and enough water, stock, or unicorn tears to cover. Cook covered, until the lentils are tender, and season with more salt and pepper if necessary. These are tasty served with mild fish like trout. Add a salad and you're done.