"Pre IPO" as a term on Heron (and elsewhere) is typically used for privately held, high growth potential companies often in sectors like tech, consumer, defense, robotics, or biology.
On Heron they are not "early stage startups", and instead are typically later stage, with valuations in the billions. They often, but not always, will have live products and may have substantial revenue. They offer high potential upside, along with high potential risk.
It is important to note that while the name has "IPO" in it, these companies do not necessarily have plans to IPO and may never IPO. You should consider investments in these companies to be long term and liquidity can only be provided as the market allows.