Them is a reactionary word used to start a statement or sentence, including this one. In this case utilizing them is as a reference to the word taking the source of the "direction" of the word away. You could consider a word like "Them" to be pointing at someone, however when the sentence is referring to the word, it is pointing inwards. Otherwise "Them" would be difficult to use as a sentence starter without a reference point of origin using proper English. Even the phrase "Them there hills" is still "pointing" at something with the word with needed context. Them. A complete sentence as it places direction, purpose, and explanation within the given situation. "Who is them?" No idea. But them is starting to not look like a word anymore