'Tris' is a very old PIC instruction, on most modern PIC's they recommend not to use it, but to write to the tris register instead - however, so far they keep including it in the parts they release.
As already mentioned it sets the direction of the port pins, by writing the value in the W register to TRIS.
Although it is not recommended for use by Microchip, I use the "TRIS" instructions whenever I need to rapidly switch a port's direction. You don't need to switch banks to do this while "movwf trisb" only works on bank1 & bank3. It is handy when interfacing to an LCD module for example.
The instruction is available on all of the PIC16 family chips. However, there are no such instructions:
There is an OPTION_REG register, which is explained in the datasheet for the particular processor - you might also look in the INCLUDE file, which actually references the name.
The OPTION instruction loads this register.
Try reading the MPASM helpfile, it explains all the questions you keep asking!. The appendix is particularly useful!.