Unit Session Percentage is the ratio of the number of Units you've sold on an ASIN in a timeframe, compared to the number of Sessions that ASIN had in that same timeframe.
Unit Session Percentage is a critical metric about your Amazon products for a couple reasons:
Amazon heavily focuses on a positive customer experience, and products that convert well (high Unit Session Percentage) seem to be products that please the customers.
Amazon likes money, and makes money when products SELL (referral fees/commissions, other fees, etc).
UPSHOT: Products with HIGH Unit Session Percentages gain the good graces of Amazon's ranking algorithms, and are also great candidates for Amazon's own remarketing around the internet. Those products with highest Unit Session Percentages are better bets for Amazon to promote in whatever ways, since they are most likely to bring Amazon greater return.