[NortySpock]

(simplifying) ARM provides verified, tested, standardized, reasonably well designed chips (logic circuits) that your company can purchase a license for and then send that chip design / logic circuit to be etched on a wafer, cut, encapsulated, and soldered to a printed circuit board.

Those ARM CPUs support a standard (but ARM-flavored) assembly programming language. (Formally: Instruction Set Architecture)

Designing your own chip previously was risky because you might have logic or hardware bugs in your chip that were very hard to debug, and then you hope that someone will bother to write assembly code that works on your chip. Since you probably designed your own assembly language that co-evolved with your chip, those assembly code developers are going to be sinking a lot of time into understanding your chips and assembly code quirks to wring performance out of them.

RISC-V standardizes a RISC-V flavored assembly code (ISA) and also provides some certification test packages to prove that "this particular chip design" can execute the RISC-V assembly language according to specifications.