You lend your money to other people through the marketplace and they pay it back with interest. The ROI is about 10% to 15% per year. I think it is low risk since you can choose the type of the loans you want to invest in (loans secured by real estate or short term loans with buy back guarantees). You can also diversify since the minimum investment is €10.

I don't think you can invest hundreds of thousands or millions of euros (the market is not so big) but the market can definitely support tens of thousands of euros.