I generally think of it as the same as "low-level programming". The wikipedia article is quite good. A classic example is a driver or operating system.
Typically in computer science people think of "systems programming" as to do with writing operating systems, device drivers, compilers and so on. So called "low level" code. Could be code in small, embedded systems, without an operating system. It implies interacting with hardware and maximizing performance and minimizing memory and other resource consumption.
But I'd like to think of "systems" as used in Rust circles as meaning "systemic". As defined as: "relating to a system, especially as opposed to a particular part". That is to say, the whole computer system, everything. From operating system device drivers to web browsers to my application code and everything in between.