Previously, we mentioned function calls and the idea of a function prologue/epilogue, as well as we told you when you would need to think about it, but not yet.
Well... here we are.
Learn what calling conventions are: https://wiki.osdev.org/Calling_Conventions
This is also a great StackOverflow answer: https://stackoverflow.com/questions/2535989/what-are-the-calling-conventions-for-unix-linux-system-calls-and-user-space-f
Explain what a calling convention is and why it matters when looking at Assembly.