I just means that people can publicly invest in the company. You could buy shares in Unity on the public market if you wanted to.
The risk is that Unity could lose the majority ownership of stock, thus the investors would sway the vote meetings when steering the company instead of Unity internally deciding this with private venture capitalist investors like they have been doing previously. The risk of this happening is very low for many years, as Unity has maintained over 50% of it's ownership with employees.
For regular developers just making stuff, this is something to ignore.
