Here's a quote from the help screen:
About site-by-site optionYou would use this option if you wanted to make it a Supporter premium plugin or you wanted to limit to specific sites. This means your users:
1. Can choose to activate their plugin on their site via Plugins -> Installed in their site admin dashboard.
2. Need to set up their own payment gateways to manage product sales.
When you use this option you also need to activate it on the main site so the global product listings will be displayed on.
I don't understand the last line, specifically "you also need to activate it on the main site so the global product listings will be displayed on."
What main site? What's global product listings? i just want to install it so each blog owner can decide if he/she wants to activate it, like I do with most of the other plug ins.