I have tried to participate in an IPO but it just told me that I needed to gain more equity, what is equity and how do I gain it. I already have double the equity that I am trying to participate in.

You need to have an equity of at least 20 million AS$ in order to participate in an IPO.

Simplifying a little bit here, but basically you build equity by making profits at the week ending date. Making losses cuts into your equity, so it is possible to actually have a negative equity (which doesn´t necessarily mean the company is financially unstable though, there are many reasons for a healthy company to have a negative equity in real life).

Oh sorry, I forgot something...  As for:

what is equity

the (again, a little simplified, but fine for AirlineSim) equity is basically your balance total minus all loans.

Thank you,