The actual formula comes from the fact that the timer (in a standard 8051) for the baud rate is an 8 bit counter that counts up from its reload value to 255 and rolls over to zero. At that time the counter automatically reloads and counts up to 255 again. The span of the count (i.e. number of clock ticks that are used to compute one cycle of the counting is 256-reload value. This is the period of the baud rate clock. The formula that you see includes some other stuff as well. The crystal frequency is obvious, and may be something like 11059200 Hertz. The counters on the 8051 are driven by a clock that is 12 times slower than the crystal frequency. In addition the actual baud rate clock fed to the UART circuit has to be 16 times faster than the actual baud rate. (This is necessary so that the receiver portion of the UART can sample fast enough to find the bits properly by synchronizing with the start bit). The output of the timer in baud rate generator mode is also divided by two before feeding it to the UART. Thus these additional factors, 12 * 16 * 2 result in the 384 divisor in the equation.
I hope this explains........if not go to the LINKS button at the left and look in the INTERESTING SITES section for three document links for the 8051 "bible". In these you can learn all manner of details related to this subject and others about the 8051.
Michael Karas