In this article RTL refers to register transfer level. It is a way of describing hardware on very low level, it uses registers for memory (which usually translates to flip-flops when/if synthesized), wires, basic arithmetic and logic operations, but terminology may slightly change based on which rtl language is being used. It can be used to design a CPU, or any ASIC (application specific integrated circuit) chip. Instructions may resemble to processor instructions, but the end result is fundamentally different. You may run a set of instructions on a processor, while what rtl describes is often synthesized and becomes the hardware itself which performs the operations (e.g. arithmetic logic unit in the cpu).