The "percentage off the 52-week high or low" refers to when a security's current price is relative to where it has traded over the last 52 weeks. This gives investors an idea of how much the security has moved in the last year and whether it is trading near the top, middle or bottom of the range.
For example, consider a stock that in the last year traded as high as $12.50, as low as $7.50, and is currently trading at $10. This means the stock is trading 20% below its 52-week high (1 – (10/12.50) = 0.20 or 20%) and 33% above its 52-week low ((10/7.50) - 1 = 0.33 or 33%). This number is calculated by finding the difference between the current price and the high or low price over the last year, then determining what percentage of the high or low this difference represents.