Hello,
There are several explanations which you can use to answer what SFI is about in a simple way.
For example, you can inform that:
1. SFI exists since 1998;
2. It is free to join;
3. There are more than 102000 products to buy;
4. It is easy to recruit the affiliates;
5. There are commissions every month;
6. You do not need much money to invest (optional); etc.
I hope that this will help...
Best Regards