What is a bi-fold bathtub door?

It is a glass tub screen whose panel folds at a hinge, so it opens inward and tucks back instead of swinging out into the room. That makes it useful where a standard hinged door would hit a wall or fixture.

Does it fit any bathtub?

It is made for tub-edge mounting, so the tub rim needs to be flat and level where the hinge and glass sit. Measure your tub opening and the wall clearance before ordering.

Is frameless glass hard to keep clean?

Frameless panels have no frame channels to trap soap and water, which helps. The glass itself still shows water spots, so squeegee it after showers and clean it regularly.

Does it keep water off the floor?

It shields the tub side it covers, but it is a single folding panel, not a full enclosure. Expect some spray at the open end, especially from a strong shower.

What is needed to install it?

It mounts to the wall and tub edge with hinges and hardware, and the wall needs solid anchoring. A level tub rim and correct drilling are the main requirements.