RISC-V (pronounced risk five), is a Free open-source Instruction Set Architecture (ISA). Other well established ISA like x86, amd64 (Intel and AMD) and ARM, are proprietary and therefore, one must pay every expensive licenses to design and build a processor using these architectures. You don't need to pay a license to build a RISC-V processor, you only need to follow the specifications. That doesn't mean the CPU design is also free, no, they stay very much the closed property of the designer, but RISC-V represents non the less, a very big step towards more transparency and technology freedom.