no_std
Rust without the standard library: just core, no operating system, and no heap unless you bring one.
Putting #![no_std] at the top of a crate links it against core instead of std. What remains is the type system, iterators, Option, Result and atomics — everything that doesn't need an OS. Gone are files, threads, networking and println!. If you want Vec and Box back, add the alloc crate and register a global allocator over a heap of your own.
The program must also define a #[panic_handler], since there is nobody to print the error to. This is the foundation of all embedded Rust on targets such as thumbv7em-none-eabihf, with RTIC and Embassy adding structure on top. The same attribute serves kernels and bootloaders — anything that runs before there is an operating system to ask for help.