A security thread is a security feature of many bank notes to protect against counterfeiting, consisting of a thin ribbon which is thread through the note's paper.
Usually, the ribbon runs vertically, and is "weaved" into the paper, so that it at some places emerges on the front side and at the remaining places at the rear side of the paper.
Usually, it is made of metal foil, but sometimes of plastic. Often, it has some text or numbers (e.g. the denomination) engraved.
If you let light shine through the bill, you can check whether it is really weaved and not printed on it by making sure that you see one continuous line. Also, check that a metal thread really has its metallic shine.
