MCLR: First pin of PIC 18F4520
MCLR - Master Clear (It is an active low pin)
MCLRE - Master Clear Enable
MCLRE = ON (It means Enable the reset function)
MCLRE = OFF (It means Disable the reset function)
TRIS is one of the register in PIC Microcontroller.
PIC 18F4520 has 5 ports such as PORTA, PORTB, PORTC, PORTD & PORTE
For example TRISA = 0; It indicates all pins of PORTA act as OUTPUT
TRISA = 1; It indicates all pins of PORTA act as INPUT