The instruction set architecture (ISA), is open source, which means there is a minimum amount of instructions (think assembly instructions) that people can use

The instruction set architecture (ISA), is open source, which means there is a minimum amount of instructions (think assembly instructions) that people can use to target/implement the functionality of that CPU architecture, similar to x86, x64, ARM or POWER. The big thing that is different than other "open" ISAs in the past is that there is an open specification / standard that isn't encumbered by restrictive licensing or patents. This allows software (think compilers) and hardware designers to target the ISA without fears of getting sued for using the ISA or having to have restrictive licenses. It can also be simple to implement, but there are commercial, highly performant cores available too, for example, sifive.