Does it come with the track and hardware?

The listing does not confirm hardware inclusions. Confirm track and pivot hardware before ordering, since a bifold needs them to hang and fold.

What opening size does an 80-inch bifold fit?

The door is 80 inches tall. Width depends on the panel width and how many panels the set has, so measure the finished opening and match it to the door's stated width.

How much floor space does a bifold save over a hinged door?

A bifold folds against the jamb instead of sweeping into the room, so it frees the floor area a hinged door would swing through. It still needs clearance for the folded stack at one side.

Is this door prehung or a slab?

It is described as a door with stile-and-rail construction, not a prehung unit. Plan for a frame, track and hardware if your opening does not already have them.

Can it be trimmed to fit?

Stile-and-rail doors can usually be trimmed at the edges, but the amount depends on the stile width. Check the manufacturer's trimming allowance before cutting.