What kind of ingredient are Mexican quelites?

I confess I've never heard of pigweed or lambs quarters, and I've lived on 3 continents

I lived in Oaxaca (30+ years ago) and never heard this word. I was also not aware of a "wild greens" tradition there, as Oaxacans seldom ate what we would call greens. Herbs like yerba santa, cilantro, and epazote were used to flavor various dishes.