They’re described in std.builtin.CallingConvention.
A function’s prologue and epilogue are bits of assembly that do things like move arguments to/from registers (or tell you where on the stack they are), set up and/or move the instruction pointer, the return address, and so on.
Since, for example “inline” is also a calling convention, they do affect your program’s behavior, but the differences in effects are usually not (and probably should not be) on the level of changing your program’s semantics. The reason they’re relevant for FFI is that Zig and the other programming language have to agree with how to call a function in order for the handshake in and out of your code to work.
You might like reading this blog post, even if a bunch of it is just flavor at the moment 
The Fil-C Optimized Calling Convention