Dividends. The average yield on stocks comprising the S&P500 is currently around 2% but has been considerably higher in the past.
The safe withdrawal rate is the maximum amount, adjusted for inflation, that historically could be withdrawn annually from an investment portfolio over a prescribed time period before the portfolio is completely exhausted. Historically, a withdrawl rate of 4%, adjusted annually for inflation, from a balanced portfolio of stock and bonds has survived around 95% of rolling 30-year periods and has therefore been considered a "safe" withdrawal rate. There are many commentators that are now predicting that a portfolio would not survive a withdrawal rate of 4% in the coming decades (given the current high valuations of both stocks and bonds).