cannot understnd progrm code....

what is "serial.begin(3600);".....can any one explain me....what this 3600?? and under whose header file begin comes.....

http://arduino.cc/en/Reference/Serial

"Serial. begin(3600);" (Note the capital "S") initialises the serial object for a bit-rate of 3600 bits per second. (a fairly unusual bit rate)

The source is in HardwareSerial.h and HardwareSerial.cpp