It's a savoury accompaniment to a roast dinner - typically you'll have a meat (beef here), roast potatoes, green/seasonal vegetables, and yorkshire puddings.
It's flour, eggs, milk, whisked together. This is essentially what makes a British pancake, but I know you guys have different kinds of pancakes to what we have. Ours tend to be thinner... but the mixes are the same. I added more eggs than usual and let the mixture sit for a good half an hour before adding the flour. They absolutely ballooned in the oven!