It's a good question, and it's frankly so complicated that I'm not sure I can explain it. Basically own capital is the operational capital that is on your company's books that your company can budget and spend. Investor money aren't.
This can play out in a lot of different ways, and many of them are probably a little different than what HN is used to, but in many cases, especially in investment banking, it's common for investors to buy into projects rather than into companies. Of course the projects are likely going to be multiple companies as legal entities, but basically, they are a lot like school projects in which one computer is great at doing the project and the other owns a computer to do the project on.
Let's say I'm really good at building offshore windmill farms. I'm also great at selling them for a nice profit once they are build, but a 40% profit on 10 is still just 14. You're rich on the other hand, but you're not really sure how to grow your fortune. Now if we pool my 10 and your 100 and the banks 100 together, then we can turn those 210 into almost 300 in the same amount of time that I could turn my 10 into 14. The 10 is own capital.
Even if you were to invest in my company directly, it would still make sense for you, and the other investors to keep some operational capital on the budgets. Both because it's healthy, and because it allows the company to move in on opportunities before they are sold to investors. Maybe there was an opportunity to turn the 210 into 1000, but you had to act NOW. I could put my own 100 up, and then slowly sell them to you, or other investors, over the following year.
And so on. It's much, much, more complicated than this, and then you also have to add the real complexity of all the bureaucratic "magic" that goes along with finances.