It's a kind of salted yeast extract. Wikipedia's "yeast extract" article[0] isn't bad, but it's missing two important points:
1. Yeast is a living organism that multiplies during beer fermentation. Commercial brewers wash and reuse some of it, but even so they always end up with excess.
2. Breaking down the protein into amino acids dramatically increases the umami taste.
So yeast extract is a way to turn a waste product into something delicious.
Things may have changed since they merged, but New Belgium Ale ran a second fermentation to create methane from the spent grains and dregs, then composted what was left. The methane from the second pass was sufficient to run some of their boilers, and the compost was consumed by the landscapers.
For anyone who doesn’t know the beer or winemaking process, I’ll put a rough overview here. It’s an interesting hobby, but one I didn’t stick with. My homes have had a lot of things but only occasionally have I had a place to make beer/wine/mead and a thermally stable place to store it (you get mouthwash if you don’t), and most of those involved narrow steep stairways I didn’t want to haul 25kg bottles up and down. I don’t think I would try again but I might help a friend if they ever wanted to.
You get two or three waste streams from brewing beer, not counting any bad batches/ingredients.
For beer, you sprout the grain in water to convert the starches to sugars, for everything, you then boil the sugary soup to release the sugars and minerals into the water and sterilize any wild yeast and bacteria. Sometime around when the yeast is added back in, the water is drained off and any solids are discarded. During the initial fermentation there are still suspended solids that will settle out, first due to lack of agitation, then flocculated by yeast grabbing onto some of what remains, but over time more and more of the dregs will be yeasts that have aged out or poisoned themselves with alcohol. For commercial processes, where space is at a premium, a food safe flocculant (something akin to chalk or clay IIRC) may be added to the end and or a mechanical filtering stage to get what’s left. But I think in these situations the tank may be agitated to speed up the yeast metabolism, trading time for complexity.
Spent grains or fruit after the boil, then one or more passes of sediment that eventually become mostly dead yeast. Brewer’s yeast is these later stages and it contains everything a microbe needs to live except sugars and oxygen.
A salty black paste made from yeast extract. Its taste is famously polarising.
It is typically spread on bread products such as toast or crumpets. It's actually quite healthy, as it's packed with B vitamins, and probably replaces a spread much higher in calories.
If you want a black substance high in minerals to add to your food, I highly recommend molasses.
Molasses are good for preventing anemia in meat poor diets (such as peasants), which is likely why it was historically common in recipes. Once upon a time there was a similar substance from sugar beets but that has largely fallen out of use.
Yeast extract is packed with umami. It compares in taste to that amazing reaction that heat has on vegetables or meat when it browns. The stuff is also quite salty packing a generous amount of flavour in such a small spread.
Like a salty, yeasty version of molasses. You’re supposed to either love or hate it but that’s largely marketing. Goes great by itself on toast, makes a wonderful bagel with cheese, and can add depth of flavour where some saltiness is needed in recipes.
