Every processor understands a particular set of basic commands, its instruction set, and historically the most common ones were owned and licensed by a few companies. An open instruction set publishes this blueprint freely, so anyone can design a chip that uses it without paying for permission.
This lowers the barrier to building custom processors, especially for specialised or low-power devices. It does not make chip design easy, which remains hard and costly, but it removes one gatekeeper and lets a wider range of designers tailor hardware to their exact needs.