'Board Support Package'. The library that FreeRTOS links to or whatnot, custom to each board shipped (a board being, SOM on a little dev card etc)

They vary massively. Usually written in a sweatshop in the far east by students, pushed out and forgotten because they are table stakes but not a profit center.

They usually include support for board boot, threads, timers, and something that looks like networking.

I say 'looks like' because they are often paper-thin implementations of a familiar API, with little or nothing inside. No proper flow control; no dynamic anything. Not even thread-safe as a rule.