Help me to understand hard money lending

[Matt Gar]

I own a duplex and a fourplex, both of which I financed through conventional mortgages and with 20% down.  I've been considering how to expand my portfolio.  In hearing/reading about how other investors expanded, those investors often talk about access to hard money.  This sound silly, but I'm not sure I understand the basics of hard money.  For example, if I locate a property that I'd like to BRR, how would hard money be utilized in that transaction?  Would a private lender lend the property purchase price and rehab costs, just purchase price, only 80% of the purchase price, etc...   Also, what are the benefits and risks of hard money?  I guess I'm just trying to understand what the options are when it comes to hard money, why folks utilize hard money, and where I can learn more.  I hope this question makes sense.