Arcoding to me . That is speed of transferring data which 8051 can do.Such as through UART of 8051 you can choose the "rate of transferring data" (baud rate)
9600 , 4800,2400...etc by control the overflowing of Timer1.
Over the serial port maximum speed is 115200 Kbit/s, but with parallel implementation over the port pins, for example using epp/ecp with uC/PC comunication, speed can be higer than 1Mbyte/s.