This is a ratio used by investors to identify a stock which is undervalued and is at a bargain price.
Basically what you do is you divide the Market capitalization (market value) of the company by its sales figure for the last 12 months. As a general rule stock trading at PSR of 1 or less than 1 is worthy of your attention.
Eg. Company ABC has a issued shares of 300 Million. Current market price is Rs. 50/=. Revenue for the period ending 31.03.2011 is Rs. 18 Billion (Rs. 18,000 Millions).
Market cap = 300 X 50 = Rs. 15,000 Millions
PSR = 15,000 / 18,000 = 0.83.
This means you can buy Rs. 1/= worth of company's sales for just 83 cents. So its at a bargain.
This can be used alongside with PER and PBR to value a stock. This is more suitable to value a company which sells tangible goods than valuing a company which offer services.