This was used in many older homes and can be recognized by its galvanized steel color. Over the years it fills with lime and corrosion and flow of water can be significantly reduced. If it must be worked on try to be as gentle as possible and do the minimum required. Pulling and twisting with a pipe wrench at one location can easily cause leaks at other locations.
Pipe can be threaded, but it is probably better to transition to one of the newer more flexible materials.