What does it mean to "tristate a pin" on a CMOS microcontroller?
"Tristate" means a state of high impedance. A pin can either pull to 0 V (sinking current, generally), pull to 5 V (sourcing current, generally), or become high impedance, like an input.
The idea is that if a pin is in high impedance state, it can be pulled to high or low by an external device without much current flow. You see this kind of thing on bidirectional serial lines, where sometimes a pin is an output and sometimes an input. When it's an input, it's "tristated," allowing the external chip to control its logic level.
Does that make sense in your situation?
The idea of tri-state is to allow multiple output devices to share a single bus. For example, multiple RAM/ROM chips can be connected to a data bus. Only the selected chip will have active outputs (high or low), the other chips (not selected) will have all their outputs set to the hi impedence state (the third state). For outputs devices to share a bus they must be tri-state capable (usually controlled by chip-enable/direction pins). For microprocessors, micro controllers their hi impendence state allows other devices to use the bus (common for smart I/O devices, DMA etc).