Knowledge is the body of facts and information surrounding a specific topic. That topic can be about processes, company policies, knowing who to speak with to get things done, or general skills. In fact, it can be about practically anything.
Knowledge has the trait of specificity. That just means that it is relevant only in certain situations. A very smart person without specific knowledge can be very ineffective. Likewise, an uneducated person who knows which dial to turn can be vital.
Knowledge in a company needs to be documented, turning tribal knowledge into something more permanent. When it is recorded, it provides more stability in processes as people move around, and protects the company from an employee leaving with a brain full of important facts.
Recording knowledge also makes it easier to share best practices, increasing opportunities for employees to help each other out, and speeding up training. Keep in mind that unrecorded knowledge has a half-life. It decays if not used.