Refer to 89s52 datasheet, Pin 3.0 & 3.1 are dedicated UART pins RXD & TXD. RXD receives serial data while TXD transmits. But there are lots of modes for that. You have to configure UART by generating specific baud rate by T2CON register, setting SCON register, reading or writing the transmitting data in SBUF register. Its easy to configure unlike I2C.
There are lot of things to know. Refer to the following.