Price Index 9m = Current share price / Share price 9 months ago.
It's a ratio that shows the return of a share price over a past period.
A ratio > 1 shows that there's been a price increase over the period whereas a ratio < 1 shows that there has been a price decrease.
Dividends are included (added) to the price index calculation.
Read more about momentum investing
You can read more about momentum investing here: 10 myths about momentum investing, squashed