How implement an IP stack?

I want to understand what implement an IP stack means.

I explain myself : I've wrote two Java little applications (Client-Server) which communicate over a LAN with TLS. In my code, I didn't have to deal with IP addressing : First I would get IP from the Server with an UDP multicast, and then I would create a SSL Socket and connect it to that IP address.

Now, I have to work on Suse, with C. And I have to implement an IP stack.

I don't understand how an IP stack can be implemented in a application, I thought it was implemented in the OS.

Can someone explain what i'm missing ?