What is the real meaning of Pre-IPO ? should we invest?

Nowadays we can see that lot of people are talking about pre-IPO which is definitely new for me.
All i just want to know whether it is safe to invest in Pre-IPO or it is just a fraudulent thing.

So pre IPO was used previously by a lot of HNI and family offices where the price of the stock might be slightly higher than the IPO price but the allotment is guaranteed. For a retail investor, this might be slightly risky in my view. If you think to get exit from a pre IPO when the company gets listed then this might also be delayed sometimes and getting an exit might become difficult. We have recently seen cases where companies have delayed their IPOs. Pre IPO investments also attract higher brokerage fees. Sometimes pre ipo investments also have lock ins which means you cant sell for a particular period. So one needs to keep these factors in mind and only then invest in such Pre IPO.