The lesson is about the importance of investing money early in life. Instead of spending money on unnecessary things like gadgets or partying, the speaker advises saving and investing money. With an initial investment of $10,000 at the age of 22, and a 10% return, one could earn $600,000 in 43 years through the power of compounding. However, waiting until the age of 32 would result in only $232,000 after 33 years, highlighting the importance of starting to invest early. The lesson emphasises the value of youth as a precious asset for investing and allowing money to grow over time.