So last day for invest knowledge, and i will be a short one as this type of investment was explained by other investment types. But it is a varied investment type and will be explained as best I can off of 3 hours of sleep.
Private investments are just as it is spelled: Private. Meaning that this investment type is one where you are investing "Privately". Examples are Angel investors for a startup, REI(Rental equity Investments) or better explained as private investment with a rental home company to get money back, private company stocks(stocks that can only be bought when working for a company), debt investment(aka buying a company's debt for them to pay you back for it with interest),investing in a film, and art collection.
Risk levels vary massively. Investing into a startup privately and it could fail. Invest in a film and it might flop in the box office. The risk and rewards are much more wild because these are private agreements rather than engaging with a company or investment opportunity through a 3rd part like the stock market or websites. While you get right to the investment you still have risks and if it fails you could lose money. It is the wildest option and one you for sure need to do the most research on. But if the investment works you can reap some good money.