Residual income is money earned from past work that continues to generate payments. For example, Michael Jackson's estate continues to earn over $200 million per year from his past music recordings and performances. Residual income can be earned through investments like rental properties, stocks, and businesses that generate ongoing royalty payments. Developing residual income is important for financial independence and security since it continues even if you stop actively working. The document discusses how joining the 5Linx company could help build $2,000 per month in residual income through marketing essential services that everyone uses.