This is a basic RS-232 transmit/receive circuit that is necessary for PIC microcontrollers to reliably communicate with a PC serial port. Why is it necessary? Well, the RS-232 specification requires that the signal levels be ยฑ3-15 Volts and unfortunately the PIC microcontroller operates at TTL levels (0-5 Volts). The MAX233 is a nice IC that allows you to convert microcontroller voltages to RS-232 compatible levels with no required external components.
A MAX233 is used to convert the logic level signals of the PIC microcontroller to RS-232 compatible voltage levels. Technically the MAX233 is a RS-232 line driver/receiver, but an easier way to explain its operation is that it allows a PIC microcontroller to communicate at RS-232 voltage levels with a computer. The MAX233 should nominally be run at 5 Volts.