TXREG is the USART Transmit Register. If you have the USART configured, and turned ON, sending a byte to this register transmits the byte via the hardware USART.

RCREG is the USART Receive Register. Bytes received by the hardware USART are stored here.

You access them with PBP the same way you would with any other internal registers.

X VAR BYTE
X = "A"
TXREG = X ' Send character in X to TX register

Y VAR BYTE
Y = RCREG ' Load received data byte into Y from RX register