collateral for the private money investors

I have few friends interested in funding my next BRRRR deal. I offered 10% annual interest which I will pay in full when I refinance the property , in about 7 months. So they get about 7% interest. They are good with that. But they want to know what collateral I will provide. How does private money funding normally work?This is only my second deal, so I am VERY new here. Would greatly appreciate any info on this topic.