What is the largest component of your total wealth? Your brokerage account? Your retirement account? For many people, the biggest part of their wealth lies in their ability to earn income in the future.
Human capital is the present value of your future labor income. Some people's jobs are like a bond. A tenured professor can reasonably rely on a fairly steady stream of income over time. Other jobs are more speculative, like stocks. Working at a high tech start-up may lead to a large pay-off, but the business could just as easily go bust.