changes a pin's data direction to input.
In assembly language, converting a pin to input requires writing a 1 to the corresponding bit of the port's TRIS register. For example, to make pin 7 of port RB an input (and the rest of RB's pins outputs), you could use the instructions mov !rb, #10000000b. (In assembly, a 0 in TRIS means makes a pin an output; a 1 makes it an input. This is the opposite of the BASIC Stamp.)
When the PIC first resets, all I/O pins are set up as inputs by default. "Input" really has two meanings: (1) the ability to read the state of pins as set by circuitry outside the PIC, and (2) disconnection of the PIC's high-current drivers to present an almost-open circuit to the outside world. The second definition applies to instructions like Pwm that perform their output function, then disconnect to avoid interfering with the operation of other circuitry. In the case of Pwm, disconnecting the output dri ver allows a capacitor to retain the voltage level set by the burst of pulse-width modulation.
According to the PIC's manufacturer, Microchip Technology Inc., the TRIS registers can be corrupted by static discharges on the I/O pins. They recommend periodically rewriting the TRIS registers, just in case.