Like a stack frame, but allocated once of a fixed size, instead of being LIFO allocated in a reserved region (which itself must be allocated upfront, when you don't know how big you're going to end up).
The difference being: if your tasks need 192B of memory each, and you spawn 10 of each, you just consumed a little less than 2kB. With green threads, you have 10 times the starting size of your stack (generally a few kB). That makes a big difference if you don't have much memory.
