Final answer:
Substantial returns from investments indicate significant profits earned over time, but they come with high risk and require careful consideration of risk-return tradeoffs.
Step-by-step explanation:
When the reading says his investments brought in substantial returns, it means that the investments made have yielded a significant profit over time. These high returns of stock market investments typically represent a high average return expected over several years or even decades. However, due to the accompanying high risk of these investments, the rate of return can vary widely in the short term, making it necessary for investors to consider the tradeoffs between risk and return based on their stage in life and financial goals.