Brown organic matter. As opposed to green. Green grass / dead dry brown grass kind of thing.
Typically contains a lot of carbon and is low in nitrogen. It provides structure to the soil, aeration and fertility.
I'd probably generalize it as dead, dry plant material. (at least in the gardener's world)
Most of the time green organic material is any fresh materials. High in nitrogen and rich in organic life.
Generally green material needs to be composted. But again there are exceptions. Comfrey mulch is one i can think of.
Also anything with high fertility like composted manure, you'd still consider to be green.
Composts that don't get enough browns take a lot longer to break down, encourage anaerobic activity and end up being heavy and silty.
Brown mulch is my go to for mj and my worm bin. Sugarcane mulch works really well for myself, so that's why i use it.
Don't get me wrong either. Parent material of mulch is still important. Large wood chips are probably the worst. Especially if it's still a bit green. (again it still has it's own useful purpose)
Bit off topic. But i was listening to a radio interview, about this new way of dealing with a human body. As opposed to conventional means like cremation / burial.
They stick your corpse in a giant compost tumbler full of mulch and compost you, bones and all in a matter of weeks. Then offer family the soil. Without all that carbon from the mulch, that process just wouldn't happen the same way. Let alone composting the bones too.
Composting is an amazing process, so fascinating really...
Little morbid way of explaining it, though it's the honest truth.