Chenopodium Album, also known as Goosefoot, Pigweed, or Lambsquarters, is a member of the amaranth family that has become endemic to much of California up to 5900 ft in elevation. It's originally native to Europe and Asia, where it has been known to be grown as a food crop for people and livestock.
Lambsquarters' distinguishing features include soft, arrowhead-shaped, gently toothed, dark green foliage that appears silvery-gray on the underside of the leaf; the leaves also can have a fine, white powdery substance dusting their surfaces; the plant is an annual that has a large, shrub-like growth habit that I've seen grow to roughly seven feet high under good conditions, though most top out around four of five feet; it has strong stems that, as the plant ages, get streaked with purple and dry to a woody texture when the plant has died; in the late summer and early fall, the plant manifests inflorescences from which very small, black seeds will eventually fall.* The plant prefers disturbed soil, and is a common sight in agricultural and untended urban settings.
*Some sources indicate that Lambsquarters can start flowering in May. I have spent a lot of time around these plants and I can't remember ever having seen that, but it's possibly down to regional variance.
Like many other invasive species endemic to California, Lambsquarters is extremely drought tolerant. This is probably one of the more aesthetically pleasing endemic invaders come August or July, as it's one of the few remaining wild plants that appears healthy, happy, and green. I have some experience foraging Lambsquarters; its flavor is just sort of an unimpressive green flavor, though the leaf has a nice bite. Apparently, the leaves are very high in protein, and I think it'd probably do swell in a soup (though, again, I'd recommend boiling the leaves in a change of water first, as some sources suggest that Lambsquarters has oxalates that'll mess you up over the long run). I have no experience using the seeds as a food source, but it seems fairly straightforward, winnowing aside. The seeds are extraordinarily small and likely won't lend themselves to milling, but would probably make for a good supplementary grain to a porridge or something like that.
So, how bad is Lambsquarters?
Well, the California Invasive Plant Council doesn't have a page dedicated to Lambsquarters. That said, it has been known to be a reservoir of viruses for crop species also in the Amaranth family. It's likely that Lambsquarters, like the other invaders from the Amaranth family, are both practically impossible to be rid of while also being generally self-limiting, in particular given the plant's preference for disturbed soils.