The various rooms in a home may have different types of flooring. At the very least the bathroom will have a something different like tiles. Quite often the kitchen is different too. The space where one type of flooring meet another type of flooring will require a joint. This joint will be covered with a piece of edging material.
If there is a door on the room then the edging material will be beneath the door, or perhaps beneath the fly screen door.
The entrances and exits in the home will also require a piece of edging material beneath the door. Sliding doors have this edging as part of the sliding mechanism. This includes retractable fly screens. Other doorways have the edging material beneath the main door or the fly screen frame
The edging strip may be made from timber or plastic, but metal is more common. The shape of the strip may accommodate a difference in flooring heights, because tiled floors tend to be a little higher than bare wood. The size of the strip may accommodate a larger gap between the two floor surfaces.
An edging strip will not protect against moisture or water spills. The floor joints beneath the strip should be sealed with silicone. The edge strip is there to look neat and prevent the edges of the carpet, tile or timber floor from being damaged.
A neat edging strip beneath the fly screen doors is part of the home design. Consider this when choosing a door and flooring materials.