The phrase 'eat well, sleep well' refers to the risk-return trade-off that most investors must make.
When investors decide which securities to buy, they also make a decision about the risk they are willing to. Buying high-risk securities offers the possibility of earning high returns and big profits ('eating well'), but buying low-risk securities offers the possibility of reliable returns ('sleeping well').
The risk-return trade-off is one of the most fundamental decisions an investor must make. Through diversification and a variety of strategies, most investors try to eat well and sleep well -- but few succeed.