A pin can be configured to output a high signal (pull towards Vdd), output a
low signal (pull towards Vss), or it could be configured as an input (let
the pin float in high impedance mode). A tristated pin means that it is
configured as an input. Once it's understood that tristate and input are
synonymous, the TRISA, TRISB, etc. register names and how the bits work seem
quite logical.
The brown-out-detect module measures the voltage of Vdd and when it drops
too low (somewhere around 3V I think for PIC12F and PIC16F series), it will
place the chip in reset. In other words, when the power supply starts
sagging, it will detect the condition, stop executing code, tristate all the
pins, and reset the program counter. When the voltage climbs back above a
certain threshold, the PIC restarts executing code (but from the beginning
since the program counter was cleared). The brown-out-detect module can be
useful if you are switching power MOSFETs or something which would explode
if you gave it a prolonged gate drive of only 3V or less.
Howard Henry Schlunder