What is "WriteCallback" in C++ for Arduino?

In a library I use, there is a WriteCallback type. I don’t know what is it, I cannot find it define anywhere ?

Could someone explain me what is WriteCallback ? Is it a native function or type in c++ ?

.h file

#if defined(ARDUINO) && ARDUINO >= 100
  #include "Arduino.h"
  #include "WConstants.h"

typedef void (*WriteCallback)  (const byte what);    // send a byte to serial port
typedef int  (*AvailableCallback)  ();    // return number of bytes available
typedef int  (*ReadCallback)  ();    // read a byte from serial port

void sendMsg (WriteCallback fSend, 
              const byte * data, const byte length);
byte recvMsg (AvailableCallback fAvailable, ReadCallback fRead, 
              byte * data, const byte length, 
              unsigned long timeout = 500);

What is "WriteCallback" in C++ for Arduino?