From the Link:
Simply put, that would mean there is at least another new high coming in the new few months (the summer rally?) before any significant bearish behavior in the stocks.
The heads up is to say those highs, if they come, will be opportunities to sell, or at least tighten stops on long-term investments. A second look at the chart shows that the SPX, coming off highs in margin debt, declines close to 50%. Those were real bear markets. The next one could be worse. Regardless, no matter how low it goes, it is best to be avoided.
There are two possibilities it could be somewhat different this time. One, margin debt itself could surge to another new high along with a strong months-long market rally (see the jingle-jangle in 2015 on the chart); or two, the top is already in and the next leg down (given how astronomically high the margin debt is beyond 2000 and 2007) could be a dead bull dropping right out of the sky (they can not fly forever).