This is the signature of any particular camera that is left on the film, usually outside the standard image area and between the perforations.
They are usually only found in camera originals of 16mm, but they can be copied onto a duplicate by printing through the margin, which is sometimes done. The presence of a key is not absolute proof that a film is a camera original. The key shape shown is a valid one but you will find there are many other shapes, some of which are an additional part of the image area (i.e. not separate as in the example above).
Please refer to an experienced technician if you have any trouble identifying a camera key. There is a diagram of all known keys which one can refer to if you need confirmation of the shape. Camera keys are usually used as evidence that a particular piece of film is an original, but not all cameras had these keys stamped into their gates. So if you suspect that a film may be a camera original (16mm) and cannot find a key you will have to corroborate your suspicions with other evidence, but the lack of a key does not mean that the stock is not a camera original.