AD Percent = (Advances Less Declines) / Total Issues
In this case we are looking at the Advance-Decline Percent for the S&P 500 which measures the degree of participation of the S&P 500 stocks. When $SPXADP is positive and advancing it shows that more and more S&P 500 stocks participate in the uptrend as was the case during the past week. Earlier, starting in June there was a divergence between the 21-day EMA and $SPX (in the background) which foreshadowed the recent pullback.